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/routes/session-routes.ts

@@ -633,6 +633,9 @@ export function registerSessionRoutes(
         }
       }
 
+      // COD-118: an explicit user-initiated start clears any tripped PTY-exit
+      // circuit breaker so an intentional restart is never blocked by a prior crash-loop.
+      session.resetRespawnBreaker();
       await session.startInteractive();
       getLifecycleLog().log({
         event: 'started',