fix: self-heal sessions evicted from the backend (Session is not active -32004)

[william0wang]

Problem

Replying to a conversation in the editor sometimes failed with:

session/subscribe failed: Session is not active: sess_… (code -32004)

while remote clients saw the same session as empty (the editor still
showed its local copy of the history).

Root cause (verified against live backend)

The zcode backend evicts idle resident runtimes after ~10 minutes
(observed log: session.resident_deactivated, reason: "idle_timeout",
idleTimeoutMs: 600000) and also keeps an LRU cap on residents. An evicted
session fails every session-scoped RPC with -32004 while the session
file stays intact:

method	evicted session	after session/resume
session/read	ERROR -32004 (fast fail)	OK
session/messages	ERROR -32004 → fetchMessages swallows → empty replay (the "remote sees empty session" symptom)	OK
session/subscribe	ERROR -32004 (the editor error)	OK
session/resume	reloads the session fully	—

The bridge trusted in-memory state forever: sessionMap returned the stale
sid from prompt(), and a stale backendLoadedSessions flag made
session/load skip the backend resume RPC.

Fix — self-healing on every entry point

• prompt(): on Session is not active from subscribe → session/resume
  reload → re-baseline the differ (so turn completion doesn't diff-replay the
  whole history) → retry the subscribe once. Other errors propagate unchanged.
• TTL on the loaded-verification (BACKEND_RESIDENT_TTL_MS = 5min, half
  the eviction window): session/load·resume stop trusting stale
  verifications and re-issue the resume RPC; timestamps refresh on subscribe
  success and turn completion.
• ensureRealSession(): config/slash/extension entry points reload a
  stale mapping before use — skipped while a turn is in flight (a running
  turn proves the resident is live), fail-safe on reload failure.

Testing

• New tests/session-eviction.test.ts (8 cases): prompt recovery + re-baseline,
  non-eviction error propagation, recovery-resume failure propagation, TTL
  expiry, stale-mapping reload, in-flight skip, fail-safe.
• Full suite: 678/678 passed; typecheck/lint/build clean.
• docs/TROUBLESHOOTING.md: dedicated -32004 entry (also corrected the
  subscribe timeout retry numbers).
• Version 0.11.3 + CHANGELOG.