refactor(hooks): tighten the hook-protocol contract surface

Implement the tighten-hook-protocol-contract RFC (moved to implemented/):

- HookDialect narrows to 'claude' | 'codex': the 'native' variant had zero
  producers (native plugins on the seams write no hook/* provenance), and the
  dialect is defined as the bridge that ran the hook.
- HookOutput.suppressOutput is gone: the codec parsed it and every path
  discarded it with no warn and no deferral — hook stdout never enters a
  transcript, so there is nothing to suppress.
- hook/result.durationMs is gone: durable timing telemetry with no reader
  that the snapshot normalizer had to scrub as replay noise. With no duration
  to measure, runHook loses its injected now clock and the single-field
  RunHookResult wrapper — it returns the HookOutput directly. The committed
  hook fixtures had the field stripped mechanically (field-only diff); the
  stdout goldens never carried it.
- The bridges' double-defaulted defaultTimeoutMs config knob is replaced by
  one reference-default constant, DEFAULT_HOOK_TIMEOUT_MS, exported from the
  lib's runner and applied inside runHook; per-hook timeoutSec stays the
  override surface.
- The hook/result semantics move into the lib that declares the event:
  HookResultRecord now carries the decoded HookOutput and appendHookResult
  derives the decision string (decision ?? stop-on-continue:false ?? pass)
  and the 500-char stderrSummary truncation; both bridges delete their
  byte-identical private copies. The snapshot suite passes against the
  existing goldens, proving the derived values are unchanged.
- Rider: BLOCKING_EXIT_CODE is codec-internal again (zero importers).

Amend the hook-protocol-lib and hook-snapshot-matrix RFCs to the new facts,
update the lib/bridge READMEs and the session.md event tables, and retarget
the affected unit tests (including new lib-level coverage of the derivation
rules).

examples/acp-agent/tests/snapshot-normalize.spec.ts

@@ -91,20 +91,14 @@ describe('normalizeSessionLog', () => {
     expect(out).toContain('{{sessionId}}')
   })
 
-  it('zeroes a hook/result durationMs (run-to-run noise) but keeps its decision', () => {
+  it('leaves event data fields untouched beyond the time zeroing (no per-event field scrubs)', () => {
     const ev = JSON.stringify({
       type: 'hook/result', seq: 2, time: 5,
-      data: { turn: 1, point: 'UserPromptSubmit', handlerId: 'h', decision: 'block', exitCode: 2, durationMs: 37 },
+      data: { turn: 1, point: 'UserPromptSubmit', handlerId: 'h', decision: 'block', exitCode: 2 },
     })
     const out = normalizeSessionLog(`${header({})}\n${ev}\n`, ctx)
-    expect(out).toContain('"durationMs":0')
-    expect(out).not.toContain('37')
-    expect(out).toContain('"decision":"block"') // the decision is the behavior — kept
-  })
-
-  it('leaves a non-hook event durationMs untouched (only hook/result is scrubbed)', () => {
-    const ev = JSON.stringify({ type: 'tool/result', seq: 2, time: 5, data: { durationMs: 88 } })
-    const out = normalizeSessionLog(`${header({})}\n${ev}\n`, ctx)
-    expect(out).toContain('"durationMs":88')
+    expect(out).toContain('"decision":"block"')
+    expect(out).toContain('"exitCode":2')
+    expect(out).toContain('"time":0')
   })
 })