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).

docs/rfc/implemented/testing/2026-07-04-hook-snapshot-matrix.md

@@ -27,7 +27,7 @@ Thirteen scenarios under `examples/acp-agent/tests/snapshots/`, naming `hook-<di
 - **Authored, no model turn** (keyless, no sidecar — the derived replay script is empty; the `rejected` turn carrying `hook/*` events is compared): `hook-cc-promptsubmit-block`, `hook-codex-promptsubmit-block`.
 - **Recorded against the real API, hook active during recording** (the model's reaction to the decision is part of the captured transcript, replayed keyless thereafter): `hook-{cc,codex}-promptsubmit-context` (allow + additionalContext fold), `hook-cc-pretool-deny` / `hook-codex-pretool-block` (deny → `isError` tool result), `hook-cc-pretool-ask` (ask → degrades to deny with the approval-required reason), `hook-{cc,codex}-posttool-block` (block with feedback), `hook-{cc,codex}-posttool-context` (accept + additionalContext), `hook-{cc,codex}-stop-continue` (a blocking Stop hook forces one extra step via steering).
 
-Each hook command emits only FIXED LITERAL strings (no timestamps/pids/`$RANDOM`/cwd echoes); the snapshot normalizer scrubs the one volatile field a `hook/result` carries (`durationMs`). The `Stop` scenarios self-limit with a marker file (`.stop_fired`) so the force-continue does not loop — the `stop_hook_active` loop-guard is still a bridge `TODO`, so an unconditional Stop hook would force-continue every step.
+Each hook command emits only FIXED LITERAL strings (no timestamps/pids/`$RANDOM`/cwd echoes), and a `hook/result` carries only deterministic fields (`decision`/`exitCode`/`stderrSummary`), so the hook events need no scrubbing beyond the normalizer's generic `time` zeroing. The `Stop` scenarios self-limit with a marker file (`.stop_fired`) so the force-continue does not loop — the `stop_hook_active` loop-guard is still a bridge `TODO`, so an unconditional Stop hook would force-continue every step.
 
 ### Three hook points are deliberately NOT snapshotted