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-cc-posttool-context/session.jsonl

@@ -59,7 +59,7 @@
 {"type":"assistant/message","seq":57,"time":1783095113150,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to run `echo HELLO` using the bash tool and report the result verbatim."},{"type":"tool-call","id":"call_00_upvgqMKJ4hJck9LxQn0p5500","name":"bash","arguments":"{\"command\": \"echo HELLO\", \"description\": \"Run echo HELLO\"}"}],"usage":{"inputTokens":72,"outputTokens":89,"cacheReadTokens":1664,"reasoningTokens":23}},"sourceEventSeqs":[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56],"surfaceOp":"append"}
 {"type":"tool/call","seq":58,"time":1783095113150,"data":{"turn":1,"step":1,"callId":"call_00_upvgqMKJ4hJck9LxQn0p5500","name":"bash","arguments":"{\"command\": \"echo HELLO\", \"description\": \"Run echo HELLO\"}"}}
 {"type":"hook/invoked","seq":59,"time":1783095113166,"data":{"turn":1,"point":"PostToolUse","dialect":"claude","handlerId":"claude:PostToolUse:1","matcher":"bash"}}
-{"type":"hook/result","seq":60,"time":1783095113175,"data":{"turn":1,"point":"PostToolUse","handlerId":"claude:PostToolUse:1","decision":"pass","exitCode":0,"durationMs":8.75206100000014}}
+{"type":"hook/result","seq":60,"time":1783095113175,"data":{"turn":1,"point":"PostToolUse","handlerId":"claude:PostToolUse:1","decision":"pass","exitCode":0}}
 {"type":"tool/result","seq":61,"time":1783095113175,"data":{"turn":1,"step":1,"callId":"call_00_upvgqMKJ4hJck9LxQn0p5500","content":[{"type":"text","text":"HELLO\n"}],"isError":false},"sourceEventSeqs":[58],"surfaceOp":"append"}
 {"type":"context/message","seq":62,"time":1783095113176,"data":{"content":[{"type":"text","text":"Note: command output has been verified against the audit log."}],"source":{"kind":"plugin","plugin":"hooks-claude"}},"surfaceOp":"append"}
 {"type":"step/end","seq":63,"time":1783095113176,"data":{"turn":1,"step":1}}