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-stop-continue/session.jsonl

@@ -30,7 +30,7 @@
 {"type":"assistant/message","seq":28,"time":1783095446572,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to reply with the single word \"FIRST\" and stop."},{"type":"text","text":"FIRST"}],"usage":{"inputTokens":55,"outputTokens":20,"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],"surfaceOp":"append"}
 {"type":"step/end","seq":29,"time":1783095446573,"data":{"turn":1,"step":1}}
 {"type":"hook/invoked","seq":30,"time":1783095446573,"data":{"turn":1,"point":"Stop","dialect":"codex","handlerId":"codex:Stop:1"}}
-{"type":"hook/result","seq":31,"time":1783095446588,"data":{"turn":1,"point":"Stop","handlerId":"codex:Stop:1","decision":"block","exitCode":2,"stderrSummary":"Also reply with the single word SECOND, then stop.","durationMs":14.944511000000148}}
+{"type":"hook/result","seq":31,"time":1783095446588,"data":{"turn":1,"point":"Stop","handlerId":"codex:Stop:1","decision":"block","exitCode":2,"stderrSummary":"Also reply with the single word SECOND, then stop."}}
 {"type":"steering/message","seq":32,"time":1783095446588,"data":{"turn":1,"content":[{"type":"text","text":"Also reply with the single word SECOND, then stop."}],"source":{"kind":"plugin","plugin":"hooks-codex"}},"surfaceOp":"append"}
 {"type":"step/start","seq":33,"time":1783095446589,"data":{"turn":1,"step":2}}
 {"type":"assistant/chunk","seq":34,"time":1783095447244,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":0,"blockType":"reasoning"}}}
@@ -243,5 +243,5 @@
 {"type":"assistant/message","seq":241,"time":1783095449029,"data":{"turn":1,"step":2,"content":[{"type":"reasoning","text":"The user asked me to reply with the single word \"FIRST\" and stop, which I already did. Now there's a steering instruction telling me to reply with \"SECOND\" and stop. However, the user's original instruction was to reply with \"FIRST\" and stop - I've already complied with that. \n\nBut the steering instruction is injected via a plugin mechanism. I should follow the latest instruction which tells me to reply with \"SECOND\" and stop.\n\nActually, let me reconsider. The original user message said \"Reply with the single word FIRST and stop.\" I already responded with \"FIRST\". Now there's a steering instruction telling me to also reply with \"SECOND\" and stop. Since the original interaction is already complete (I already said FIRST and stopped), I think the steering is trying to get me to say something additional. But the user's original instruction said to stop after FIRST.\n\nI'll follow the steering instruction since it's the most recent directive."},{"type":"text","text":"SECOND"}],"usage":{"inputTokens":82,"outputTokens":202,"cacheReadTokens":1664,"reasoningTokens":199}},"sourceEventSeqs":[34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240],"surfaceOp":"append"}
 {"type":"step/end","seq":242,"time":1783095449029,"data":{"turn":1,"step":2}}
 {"type":"hook/invoked","seq":243,"time":1783095449029,"data":{"turn":1,"point":"Stop","dialect":"codex","handlerId":"codex:Stop:2"}}
-{"type":"hook/result","seq":244,"time":1783095449039,"data":{"turn":1,"point":"Stop","handlerId":"codex:Stop:2","decision":"pass","exitCode":0,"durationMs":9.63256199999978}}
+{"type":"hook/result","seq":244,"time":1783095449039,"data":{"turn":1,"point":"Stop","handlerId":"codex:Stop:2","decision":"pass","exitCode":0}}
 {"type":"turn/end","seq":245,"time":1783095449039,"data":{"turn":1,"reason":{"kind":"completed"}}}