COD-118 feat: circuit breaker bounding repeated non-zero interactive-PTY exits

Defense-in-depth after COD-115. If the interactive PTY exits non-zero
repeatedly within a short window, recovery/reconnect paths recreate it
indefinitely (COD-115 saw 114 'exited with code: 1' events + orphans).

- New pure InteractivePtyExitBreaker (session-pty-exit-breaker.ts):
  injectable time, sliding window, clean-exit resets counter, stays
  tripped until reset(). Defaults: threshold 5, window 10s.
- Session records each interactive PTY exit in the breaker; on trip it
  flips _status to 'error', sets _respawnBlocked, emits
  respawnBreakerTripped. startInteractive() refuses to respawn while
  blocked, so all recovery/reconnect callers stop looping uniformly.
- Explicit user restart (POST /api/sessions/:id/interactive) calls
  resetRespawnBreaker() so intentional restarts are never blocked.
- New SSE event session:respawnBreakerTripped wired in sse-events.ts +
  constants.js (registries in sync) + session-listener-wiring.ts;
  minimal diagnostic toast in app.js.
- Tests: test/respawn-pty-breaker.test.ts (pure trip/reset/window +
  MockSession session-level trip/reset).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/web/public/constants.js

@@ -262,6 +262,7 @@ const SSE_EVENTS = {
   SESSION_LIMIT_PAUSE_SCHEDULED: 'session:limitPauseScheduled',
   SESSION_LIMIT_RESUME: 'session:limitResume',
   SESSION_LIMIT_RESUME_CANCELLED: 'session:limitResumeCancelled',
+  SESSION_RESPAWN_BREAKER_TRIPPED: 'session:respawnBreakerTripped',
   SESSION_CLI_INFO: 'session:cliInfo',
   SESSION_MESSAGE: 'session:message',
   SESSION_INTERACTIVE: 'session:interactive',