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/snapshots/hook-codex-pretool-block/session.jsonl

@@ -54,7 +54,7 @@
 {"type":"assistant/message","seq":52,"time":1783095409582,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to run a simple bash command and report the result verbatim."},{"type":"tool-call","id":"call_00_d1KxP9oXmTPECtwtxuVc9576","name":"bash","arguments":"{\"command\": \"echo HELLO\", \"description\": \"Print HELLO to stdout\"}"}],"usage":{"inputTokens":72,"outputTokens":84,"cacheReadTokens":1664,"reasoningTokens":17}},"sourceEventSeqs":[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51],"surfaceOp":"append"}
 {"type":"tool/call","seq":53,"time":1783095409582,"data":{"turn":1,"step":1,"callId":"call_00_d1KxP9oXmTPECtwtxuVc9576","name":"bash","arguments":"{\"command\": \"echo HELLO\", \"description\": \"Print HELLO to stdout\"}"}}
 {"type":"hook/invoked","seq":54,"time":1783095409583,"data":{"turn":1,"point":"PreToolUse","dialect":"codex","handlerId":"codex:PreToolUse:1","matcher":"bash"}}
-{"type":"hook/result","seq":55,"time":1783095409598,"data":{"turn":1,"point":"PreToolUse","handlerId":"codex:PreToolUse:1","decision":"block","exitCode":2,"stderrSummary":"bash is disabled by codex policy in this session","durationMs":15.254155000000083}}
+{"type":"hook/result","seq":55,"time":1783095409598,"data":{"turn":1,"point":"PreToolUse","handlerId":"codex:PreToolUse:1","decision":"block","exitCode":2,"stderrSummary":"bash is disabled by codex policy in this session"}}
 {"type":"tool/result","seq":56,"time":1783095409599,"data":{"turn":1,"step":1,"callId":"call_00_d1KxP9oXmTPECtwtxuVc9576","content":[{"type":"text","text":"Error: bash is disabled by codex policy in this session"}],"isError":true},"sourceEventSeqs":[53],"surfaceOp":"append"}
 {"type":"step/end","seq":57,"time":1783095409599,"data":{"turn":1,"step":1}}
 {"type":"step/start","seq":58,"time":1783095409599,"data":{"turn":1,"step":2}}