Keep state and keys out of the shared repo

The dev VM is treated as untrusted, and the repo is a read-write virtiofs
mount shared with the backend host -- so a CA private key sitting in it is
a key that machine can sign with, and a leaf signed by this CA is one the
phone's pinned app accepts without question. Pinning against a CA the
attacker holds is no pinning at all.

So certificates are now generated on the machine that serves them, into
$XDG_CONFIG_HOME/ai-app/certs at 0700 with 0600 keys (AI_APP_CERTS
overrides), and config.json and session transcripts move to the XDG config
and data directories. Transcripts move for a plainer reason than the keys:
they are whole conversations, and they were world-readable at 0644.

Two smaller things fall out. The host and VM stop sharing one config,
which had already put a test token on the production backend. And state
stops living where `git clean -xdf` would take the enrollment and every
transcript with it.

State that predates the move is still read from the repo, with a warning
naming where to move it, so an existing install keeps working rather than
silently coming up on an empty config -- the precedence is covered by a
test, since picking the wrong file would otherwise be silent.

Verified: 31 tests, clippy clean; the certificate script writing 0700/0600
into an overridden directory; and the server logging the fallback and
serving from it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

server/src/session/transcript.rs

@@ -8,6 +8,7 @@
 
 use std::fs::{File, OpenOptions};
 use std::io::{BufRead, BufReader, Write};
+use std::os::unix::fs::OpenOptionsExt;
 use std::path::Path;
 
 use anyhow::{Context, Result};
@@ -36,9 +37,12 @@ impl Transcript {
     /// the last line if one exists.
     pub fn open(path: &Path) -> Result<Self> {
         let last_seq = last_seq(path)?;
+        // Owner-only: a transcript is the whole conversation, including
+        // whatever the session read, wrote, or was told.
         let file = OpenOptions::new()
             .create(true)
             .append(true)
+            .mode(0o600)
             .open(path)
             .with_context(|| format!("open transcript {}", path.display()))?;
         Ok(Self { file, next_seq: last_seq + 1 })