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/session-pty-exit-breaker.ts

@@ -0,0 +1,102 @@
+/**
+ * @fileoverview Circuit breaker bounding repeated non-zero interactive-PTY exits (COD-118).
+ *
+ * Defense-in-depth after COD-115: if the interactive PTY exits non-zero repeatedly,
+ * external recovery/reconnect paths recreate it indefinitely (COD-115 observed 114
+ * `exited with code: 1` events + orphan sessions). This breaker tracks recent
+ * non-zero exits within a sliding window and "trips" once they exceed a threshold,
+ * so the Session can refuse to respawn and surface an error state instead of looping.
+ *
+ * Design notes:
+ * - PURE + dependency-free. Time is INJECTED (`nowMs` passed to `recordExit`); the
+ *   breaker never calls `Date.now()` itself, so trip/window logic is deterministically
+ *   unit-testable with no real timers.
+ * - A clean (exit code 0) exit resets the counter — a session that exited normally is
+ *   not on a crash-loop. (It does NOT clear an already-tripped breaker; only an explicit
+ *   `reset()` — e.g. a user-initiated restart — does that.)
+ * - Once tripped, stays tripped until `reset()`.
+ *
+ * @consumedby session (instantiates one per session; records exits in the interactive
+ *   PTY `onExit` handler; gates `startInteractive()` when tripped; `reset()` on restart)
+ * @module session-pty-exit-breaker
+ */
+
+/** Non-zero interactive-PTY exits within the window required to trip the breaker. */
+export const DEFAULT_BREAKER_THRESHOLD = 5;
+
+/** Sliding window (ms) over which non-zero exits accumulate toward the threshold. */
+export const DEFAULT_BREAKER_WINDOW_MS = 10_000;
+
+export interface InteractivePtyExitBreakerOptions {
+  /** Trip after this many non-zero exits within `windowMs` (default 5). */
+  threshold?: number;
+  /** Sliding window length in ms (default 10_000). */
+  windowMs?: number;
+}
+
+export interface RecordExitResult {
+  /** True once the breaker has tripped (stays true until `reset()`). */
+  tripped: boolean;
+  /** Number of non-zero exits currently inside the window. */
+  count: number;
+}
+
+/**
+ * Sliding-window counter that trips on rapid repeated non-zero exits.
+ *
+ * 5 within 10s safely clears normal usage (a single exit, an intentional restart)
+ * while tripping fast on a real loop — COD-115 saw 114 exits, far above 5.
+ */
+export class InteractivePtyExitBreaker {
+  private readonly _threshold: number;
+  private readonly _windowMs: number;
+
+  /** Timestamps (ms, injected) of recent non-zero exits, oldest first. */
+  private _exitTimes: number[] = [];
+
+  private _tripped = false;
+
+  constructor(opts: InteractivePtyExitBreakerOptions = {}) {
+    this._threshold = opts.threshold ?? DEFAULT_BREAKER_THRESHOLD;
+    this._windowMs = opts.windowMs ?? DEFAULT_BREAKER_WINDOW_MS;
+  }
+
+  /** Whether the breaker has tripped (respawn should be blocked). */
+  get tripped(): boolean {
+    return this._tripped;
+  }
+
+  /**
+   * Record a PTY exit. A zero (clean) exit resets the non-zero counter; a non-zero
+   * exit is added to the window, stale entries are evicted, and the breaker trips
+   * once the in-window count reaches the threshold.
+   *
+   * @param exitCode the PTY exit code (0 = clean)
+   * @param nowMs injected current time in ms (never read from a real clock)
+   */
+  recordExit(exitCode: number, nowMs: number): RecordExitResult {
+    if (exitCode === 0) {
+      // Clean exit: a normal stop, not a crash-loop. Clear accumulated non-zero
+      // exits. Does NOT un-trip an already-tripped breaker (only reset() does).
+      this._exitTimes = [];
+      return { tripped: this._tripped, count: 0 };
+    }
+
+    // Evict exits strictly older than the window, then record this one.
+    const cutoff = nowMs - this._windowMs;
+    this._exitTimes = this._exitTimes.filter((t) => t > cutoff);
+    this._exitTimes.push(nowMs);
+
+    if (this._exitTimes.length >= this._threshold) {
+      this._tripped = true;
+    }
+
+    return { tripped: this._tripped, count: this._exitTimes.length };
+  }
+
+  /** Clear the tripped state and the non-zero counter (e.g. on intentional restart). */
+  reset(): void {
+    this._exitTimes = [];
+    this._tripped = false;
+  }
+}