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).

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

@@ -21,7 +21,7 @@
 import type { HookOutput } from './types.ts'
 
 /** The exit code a hook uses to signal a blocking error (stderr → model). */
-export const BLOCKING_EXIT_CODE = 2
+const BLOCKING_EXIT_CODE = 2
 
 /** Read a string field from a parsed object, or `undefined` if absent/wrong type. */
 function str(obj: Record<string, unknown>, key: string): string | undefined {
@@ -72,7 +72,7 @@ function permissionDecisionOf(value: string | undefined): HookOutput['decision']
  * `additionalContext`/`updatedInput`) are DISCARDED — a `PreToolUse` block on a
  * `Stop` hook must not deny the `Stop`. The block's `hookEventName` is still
  * surfaced (for the log/diagnostics), and the event-agnostic top-level fields
- * (`decision`/`reason`/`continue`/`stopReason`/`suppressOutput`/`systemMessage`)
+ * (`decision`/`reason`/`continue`/`stopReason`/`systemMessage`)
  * are unaffected. Omit `expectedEventName` (or pass a matching one) to apply the
  * block as-is — a caller that doesn't key by event opts out of the check.
  */
@@ -125,8 +125,6 @@ function applyStructured(output: HookOutput, parsed: Record<string, unknown>, ex
   if (cont !== undefined) output.continue = cont
   const stopReason = str(parsed, 'stopReason')
   if (stopReason !== undefined) output.stopReason = stopReason
-  const suppress = bool(parsed, 'suppressOutput')
-  if (suppress !== undefined) output.suppressOutput = suppress
   const sysMsg = str(parsed, 'systemMessage')
   if (sysMsg !== undefined) output.systemMessage = sysMsg