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/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'