Let sessions outlive the backend, and never resume one twice

Three `claude` processes ended up running against this checkout on
2026-08-29, and the account hit its session limit. One cause, several
ways in.

An agent imported the Claude Code session it was *itself* running in.
That is an ordinary import, and importing runs `--resume` -- so a second
CLI attached to a file the first was still writing. The whole 65 MB
conversation, 154 embedded screenshots included, was re-appended to the
transcript under a new prompt id; both copies then read each other's
writes as work done elsewhere, and the adopted one was billed for
re-reading all of it. Meanwhile `shutdown_all` asked each session to stop
and the process exited immediately, so the SIGKILL timer died with the
runtime, the stop was unreliable, and whatever survived was orphaned with
nothing written down to find it by.

The processes leaked either way. So leak them on purpose, and be able to
pick them back up.

A session's process now outlives the backend and is adopted again on the
way up, which is worth having for its own sake: restarting the server no
longer ends a turn somebody is waiting on. Its stdio lives in the session
directory -- a fifo opened read-write so the process is its own last
writer and never reads EOF, plus stdout/stderr logs read from a byte
offset. `session::process` records the pid *and* the kernel's start time
for it, because a pid alone is reused and adopting a stranger's would mean
never resuming the real conversation.

That makes the fix structural rather than a check: everything goes through
`ClaudeDriver::launch`, which adopts if it can and starts if it cannot,
and `--resume` is reachable only on the second path. `Driver` gains two
ways out where it had one -- `detach` (coming back) and `stop` (the
session is being deleted, so the process must not survive).

Importing a session that is open is now refused outright. Claude Code
keeps `~/.claude/sessions/<pid>.json` for every live session, so this is a
measurement rather than a guess; it reports no/yes/unknown, because a
machine that keeps no such record cannot answer and "could not check" is
not "nobody is using it". `SessionStatus` gains `Unknown` for the same
reason.

Also here, found on the way:

- A reconnecting phone was sent the entire backlog. Opening a session was
  bounded to a page but reconnecting was not, so a long disconnect
  delivered thousands of events one frame at a time. Past `CATCH_UP_LIMIT`
  the stream sends a `reset` frame and the newest window, and the client
  rebuilds from it as it does on open -- without the reset the window is
  spliced onto rows no longer adjacent to it.
- A session's status was assumed idle at launch. Read from the transcript
  instead, so a restart stops claiming an exited session is waiting for
  you.
- `llama-server`'s stdout was piped and never drained, so a chatty one
  blocked on a full pipe buffer mid-load. It goes to a log now.
- A turn that exited or errored never emitted `Idle`, so the queue stayed
  "running" for good: every later message was held forever and, since a
  message is only recorded when taken, vanished with nothing on screen.
- Two doc comments had drifted onto the wrong functions.

Verified by killing the server mid-turn: the process survived, finished
its turn unattended (12.8 KB of output nothing was reading), and the
restarted server adopted it -- one process, all 700 lines in the
transcript, no hole, and it still took a new message afterwards. Deleting
a session stops its process; a 266-event backlog resets while a 16-event
one streams. 46 tests, clippy and rustfmt clean, app compiles and lints.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VETa8afmpWaYezLCqJhDB8

server/src/routes.rs

@@ -11,7 +11,9 @@
 //! DELETE /setups/{id}                  remove, refused while sessions use it
 //! GET    /sessions                     list (id, provider, title, model, status, last activity)
 //! POST   /sessions                     spawn {setup, provider, title?, model?, cwd?, params?}
-//! GET    /sessions/{id}/events?after=N SSE: transcript replay from N, then live
+//! GET    /sessions/{id}/events?after=N SSE: backlog after N, then live
+//!                                     (a backlog past CATCH_UP_LIMIT arrives as a
+//!                                      `reset` frame plus the newest window)
 //! POST   /sessions/{id}/message        {text, attachmentIds?}
 //! POST   /sessions/{id}/answer         {questionId, answer}   (questions and permissions)
 //! POST   /sessions/{id}/interrupt
@@ -29,7 +31,7 @@
 //! branch on the session kind (that's what drivers are for).
 
 use std::convert::Infallible;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
 use anyhow::Context;
@@ -45,7 +47,7 @@ use tokio::sync::{broadcast, mpsc};
 use tokio_stream::StreamExt;
 use tokio_stream::wrappers::ReceiverStream;
 
-use crate::session::transcript::{SeqEvent, read_after};
+use crate::session::transcript::{CATCH_UP_LIMIT, CatchUp, SeqEvent, catch_up};
 use crate::session::{LiveSession, SessionInfo, SessionManager, SpawnSpec};
 
 pub fn router(manager: Arc<SessionManager>) -> Router {
@@ -462,6 +464,20 @@ async fn spawn_session(
                      only this app's view of it."
                 )));
             }
+            // Refused rather than warned about, because there is nothing
+            // useful on the other side of it. Importing an open session
+            // puts a second `--resume` on a file the first one is still
+            // writing: the conversation gets duplicated into it, each copy
+            // replays the other's writes as work done elsewhere, and the
+            // adopted one is billed for re-reading the whole thing. On
+            // 2026-08-29 that was 65 MB and 154 screenshots.
+            if chosen.in_use == crate::session::import::InUse::Yes {
+                return Err(ApiError::BadRequest(format!(
+                    "{want} is open in a terminal right now. Importing it would put a second \
+                     Claude Code on the same conversation, which duplicates it and re-reads the \
+                     whole thing. Close it there first, then import it here."
+                )));
+            }
             let events = crate::session::import::replay(&transport, &chosen.path)
                 .await
                 .map_err(bad_request)?;
@@ -802,23 +818,8 @@ async fn stream_session(
     mut live: broadcast::Receiver<SeqEvent>,
     tx: mpsc::Sender<SseEvent>,
 ) {
-    // Synchronous file reads from an async task: transcript lines are
-    // small and local; revisit if daily use produces transcripts where
-    // this shows (phase 6 territory).
-    let catch_up = |after: u64| match read_after(&transcript, after) {
-        Ok(entries) => Some(entries),
-        Err(err) => {
-            tracing::error!("transcript replay failed: {err:#}");
-            None
-        }
-    };
-
-    let Some(replay) = catch_up(last) else { return };
-    for entry in replay {
-        last = entry.seq;
-        if send_event(&tx, &entry).await.is_err() {
-            return;
-        }
+    if !send_backlog(&transcript, &mut last, &tx).await {
+        return;
     }
     loop {
         match live.recv().await {
@@ -832,12 +833,8 @@ async fn stream_session(
                 }
             }
             Err(broadcast::error::RecvError::Lagged(_)) => {
-                let Some(missed) = catch_up(last) else { return };
-                for entry in missed {
-                    last = entry.seq;
-                    if send_event(&tx, &entry).await.is_err() {
-                        return;
-                    }
+                if !send_backlog(&transcript, &mut last, &tx).await {
+                    return;
                 }
             }
             Err(broadcast::error::RecvError::Closed) => return,
@@ -845,6 +842,45 @@ async fn stream_session(
     }
 }
 
+/// Sends everything after `last`, advancing it, and answers whether the
+/// subscriber is still there.
+///
+/// A [`CatchUp::Restart`] is preceded by the `reset` frame that tells the
+/// client to drop what it holds. Without it the window would be spliced
+/// onto rows that are no longer adjacent to it, which reads as ordinary
+/// output rather than as a gap -- which is why a bounded backlog cannot
+/// simply be "the newest events".
+///
+/// Both ways into a backlog come through here -- the first replay and the
+/// recovery from a lapped broadcast -- because either can be arbitrarily
+/// far behind and owes the client the same answer.
+///
+/// Synchronous file reads from an async task: transcript lines are small
+/// and local; revisit if daily use produces transcripts where this shows
+/// (phase 6 territory).
+async fn send_backlog(transcript: &Path, last: &mut u64, tx: &mpsc::Sender<SseEvent>) -> bool {
+    let entries = match catch_up(transcript, *last, CATCH_UP_LIMIT) {
+        Ok(CatchUp::Continue(entries)) => entries,
+        Ok(CatchUp::Restart(entries)) => {
+            if tx.send(SseEvent::default().event("reset")).await.is_err() {
+                return false;
+            }
+            entries
+        }
+        Err(err) => {
+            tracing::error!("transcript replay failed: {err:#}");
+            return false;
+        }
+    };
+    for entry in entries {
+        *last = entry.seq;
+        if send_event(tx, &entry).await.is_err() {
+            return false;
+        }
+    }
+    true
+}
+
 async fn send_event(
     tx: &mpsc::Sender<SseEvent>,
     entry: &SeqEvent,