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/session/transport.rs

@@ -20,6 +20,7 @@
 //! second is a no-op locally. See PLAN.md's SSH section.
 
 use std::path::{Path, PathBuf};
+use std::process::Stdio;
 
 use anyhow::{Context, Result};
 use tokio::process::Child;
@@ -47,6 +48,27 @@ impl Launch {
     }
 }
 
+/// How a launched process's standard streams are connected.
+///
+/// The choice is not the transport's and not the driver's dialect: it is
+/// whether the process is expected to outlive this server. A probe is
+/// asked a question and answers within one call, so pipes this server
+/// drains are right and dying with it is right. A session is a
+/// conversation somebody is having, so its streams live in the session
+/// directory where a later run of this server can pick them up again --
+/// see `session::process`.
+pub enum Streams {
+    /// Pipes owned by this server; the child is killed when they drop.
+    Piped,
+    /// Files -- and, for stdin, a fifo the child itself holds open so it
+    /// never reads EOF -- that outlast this process.
+    Detached {
+        stdin: Stdio,
+        stdout: Stdio,
+        stderr: Stdio,
+    },
+}
+
 /// The machine a session's process runs on.
 pub enum Transport {
     /// The machine this server is running on.
@@ -70,31 +92,53 @@ impl Transport {
         }
     }
 
-    /// Starts `launch`, with stdio piped and the child killed on drop.
+    /// Starts `launch` with its streams connected as `streams` says.
     ///
     /// The failure names what to check, and the two transports fail for
     /// genuinely different reasons -- a missing ssh client here versus a
     /// program that is not on the remote PATH -- so each says its own
     /// thing rather than one message hedging between them.
-    pub fn spawn(&self, launch: &Launch) -> Result<Child> {
+    pub fn spawn(&self, launch: &Launch, streams: Streams) -> Result<Child> {
         let host = match self {
             Self::Here => None,
             Self::Ssh { ssh, .. } => Some(ssh),
         };
-        crate::ssh::command(host, &launch.program, &launch.args, launch.cwd.as_deref())
-            .spawn()
-            .with_context(|| match self {
-                Self::Ssh { name, .. } => format!(
-                    "couldn't start ssh to run \"{}\" on {name} -- is the ssh client installed \
+        let mut command =
+            crate::ssh::command(host, &launch.program, &launch.args, launch.cwd.as_deref());
+        match streams {
+            Streams::Piped => {
+                command
+                    .stdin(Stdio::piped())
+                    .stdout(Stdio::piped())
+                    .stderr(Stdio::piped())
+                    .kill_on_drop(true);
+            }
+            Streams::Detached {
+                stdin,
+                stdout,
+                stderr,
+            } => {
+                command.stdin(stdin).stdout(stdout).stderr(stderr);
+                // No `kill_on_drop`: outliving this server is the point.
+                // Its own process group as well, so a signal sent to the
+                // server's group -- which is how a terminal or a
+                // supervisor stops it -- does not travel to a session that
+                // is meant to survive being stopped.
+                command.process_group(0);
+            }
+        }
+        command.spawn().with_context(|| match self {
+            Self::Ssh { name, .. } => format!(
+                "couldn't start ssh to run \"{}\" on {name} -- is the ssh client installed \
                      here?",
-                    launch.program,
-                ),
-                Self::Here => format!(
-                    "couldn't run \"{}\" on this machine -- is it installed and on PATH? If it \
+                launch.program,
+            ),
+            Self::Here => format!(
+                "couldn't run \"{}\" on this machine -- is it installed and on PATH? If it \
                      lives on another machine, give the session a host to run on.",
-                    launch.program,
-                ),
-            })
+                launch.program,
+            ),
+        })
     }
 
     /// How to say where this runs, for a log line a person reads.