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.ts

@@ -8,8 +8,7 @@
  * Scrubbed: `randomUUID()` session ids → `{{sessionId}}`; the temp `mkdtemp`
  * cwd → `{{cwd}}` (it appears in terminal-card `_meta` and the log header);
  * JSON-RPC request `id` → a stable per-transcript sequence; the log's per-event
- * `time` (epoch ms) and header `createdAt` → 0; a `hook/result` event's
- * `durationMs` (wall-clock hook runtime) → 0. NOT scrubbed: the log's `seq`
+ * `time` (epoch ms) and header `createdAt` → 0. NOT scrubbed: the log's `seq`
  * (deterministic — `seq = log.length`, part of the event-log contract).
  *
  * See docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md.
@@ -98,13 +97,6 @@ export function normalizeSessionLog(rawLog: string, ctx: NormalizeContext): stri
     } else if ('time' in record) {
       // Event line: zero the epoch-ms timestamp; keep seq (deterministic).
       record.time = 0
-      // A hook/result carries the hook's wall-clock runtime (`data.durationMs`),
-      // which is run-to-run noise like `time` — zero it so the golden reflects
-      // the hook's decision/exit, not how long the shell took.
-      if (record.type === 'hook/result' && record.data !== null && typeof record.data === 'object') {
-        const data = record.data as Record<string, unknown>
-        if ('durationMs' in data) data.durationMs = 0
-      }
     }
     return scrubValue(record, ctx) as Record<string, unknown>
   })