fix(hooks): drain detached hook runs on bridge dispose

The emit-shaped hook points (SessionStart, SubagentStart, SubagentStop)
run fire-and-forget: no seam awaits the run chain, so disposing a bridge
could strand a live hook process and let a late continuation inject into
a disposed context. The floating continuation also made the coverage
gate racy: the only coverage of the SubagentStart continuation's
no-context branch arm rode on an un-awaited .then, and on a loaded CI
runner the fork's per-file coverage snapshot beat it — master run
28798191671 failed the 100% branch gate on hooks-claude/src/index.ts at
99.03% (uncovered line 336) with the identical tree passing the PR run
three minutes earlier.

New shared primitive createDetachedRuns() in dsh-hook-protocol: a bridge
tracks each detached run chain, passes the tracker's abort signal to
runHook, and registers drain() as its effect disposer — drain aborts
still-running hook processes (a kill via the bash seam, not a wait out
to the 10-minute default hook timeout), then resolves once every chain
has settled. fiber.dispose() resolving now means the bridge's detached
work is quiescent (docs/defensive-patterns.md).

The subagent marker test disposes the bridge as its sync point, so the
formerly racy branch arm is executed deterministically before the file's
coverage snapshot; new tests pin abort-on-dispose promptness for both
bridges and the tracker's settle/drain contract in hook-protocol.

packages/hooks/hook-protocol/src/detached.ts

@@ -0,0 +1,71 @@
+/**
+ * Quiescence tracking for a bridge's DETACHED hook runs. The waterfall-shaped
+ * hook points (`UserPromptSubmit`, `PreToolUse`, …) are awaited by their seams,
+ * but the emit-shaped points (`SessionStart`, `SubagentStart`, `SubagentStop`)
+ * run fire-and-forget: no seam awaits them, so without tracking a bridge's
+ * disposal could strand a live hook process and let a late continuation fire
+ * into a disposed context (docs/defensive-patterns.md: dispose must reach
+ * quiescence). A bridge creates one tracker in `apply()`, passes
+ * {@link DetachedRuns.signal} to each detached {@link runHook} call, wraps the
+ * full run chain (the hook run PLUS its `.then` continuation) in
+ * {@link DetachedRuns.track}, and registers {@link DetachedRuns.drain} as its
+ * disposer.
+ *
+ * @module @deepseek-ai/dsh-hook-protocol/detached
+ */
+
+/** In-flight registry for one bridge's detached hook runs; see the module doc for the wiring contract. */
+export interface DetachedRuns {
+  /**
+   * The abort signal every tracked run must hand to {@link runHook} (via its
+   * `signal` option). {@link drain} fires it so a still-running hook process is
+   * killed rather than awaited out to its timeout (default 10 minutes).
+   */
+  readonly signal: AbortSignal
+  /**
+   * Register one detached run until it settles. Pass the FULL chain — the hook
+   * run and its continuation/error handler — so {@link drain} waits for the
+   * side effects (an inject, a warn), not just the process exit. A rejected
+   * chain is absorbed here (settlement bookkeeping only), but rejection
+   * handling is still the caller's job: an untracked `.catch` is what turns a
+   * failure into a logged warning instead of silence.
+   * @param run - the detached run chain to hold until settled.
+   */
+  track(run: Promise<unknown>): void
+  /**
+   * Abort {@link signal}, then resolve once every tracked chain has settled —
+   * including chains tracked while the drain is in progress. The bridge
+   * registers this as its effect disposer; cordis awaits it, so
+   * `fiber.dispose()` resolving means the bridge's detached work is quiescent.
+   * A run tracked AFTER drain resolves is not awaited by anyone — by then the
+   * bridge's listeners are disposed, so nothing can start one.
+   * @returns resolves when all tracked runs have settled.
+   */
+  drain(): Promise<void>
+}
+
+/**
+ * Create a {@link DetachedRuns} tracker (one per bridge `apply()`); settled
+ * runs are pruned so a long-lived session does not accumulate them.
+ * @returns the tracker.
+ */
+export function createDetachedRuns(): DetachedRuns {
+  const inflight = new Set<Promise<unknown>>()
+  const controller = new AbortController()
+  return {
+    signal: controller.signal,
+    track(run: Promise<unknown>): void {
+      inflight.add(run)
+      const settled = (): void => { inflight.delete(run) }
+      void run.then(settled, settled)
+    },
+    async drain(): Promise<void> {
+      controller.abort(new Error('hook bridge disposed'))
+      // Re-check after each wave: a chain can be tracked while a prior wave is
+      // settling; loop until the registry is observed empty.
+      while (inflight.size > 0) {
+        await Promise.allSettled([...inflight])
+      }
+    },
+  }
+}

packages/hooks/hook-protocol/src/index.ts

@@ -15,6 +15,8 @@
  *   session-event helpers (declaration-merged into `SessionEventMap`);
  *   `appendHookResult` derives the durable `decision`/`stderrSummary` from the
  *   {@link HookOutput} so the shared event's semantics live in one place.
+ * - {@link createDetachedRuns} — quiescence tracking for the fire-and-forget
+ *   hook points: disposal aborts and drains a bridge's detached runs.
  *
  * Each bridge owns what genuinely DIFFERS: building the per-event stdin payload
  * (CC vs Codex field sets), the dialect's env/substitution, and mapping the
@@ -38,3 +40,5 @@ export { mergeHookOutputs } from './merge.ts'
 export type { MergedDecision, MergedHookOutcome } from './merge.ts'
 export { appendHookInvoked, appendHookResult, DEFAULT_STDERR_SUMMARY_MAX_CHARS, summarizeStderr } from './events.ts'
 export type { HookInvocation, HookResultRecord } from './events.ts'
+export { createDetachedRuns } from './detached.ts'
+export type { DetachedRuns } from './detached.ts'