restore hook/result durationMs — review keeps wall-clock audit timing durable

Reverses item 3 of the tighten-hook-protocol-contract RFC per review:
a persistence log is written for future readers, and hook wall-clock
runtime is audit signal (which hook made a turn slow). runHook keeps
its injected now clock and RunHookResult wrapper, the bridges pass the
measured duration through HookResultRecord, the snapshot normalizer
keeps its replay scrub, and the hook fixtures carry the field again.
The RFC records the reversal; the other three prunes stand.

examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-context/session.jsonl

@@ -1,7 +1,7 @@
 {"type":"session","version":0,"id":"09a657ad-72c0-4fd3-aae1-00b66dfdb3d5","createdAt":1783095399158,"cwd":"/tmp/acp-snap-cwd-rBmbYp"}
 {"type":"turn/start","seq":0,"time":1783095399163,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"hook/invoked","seq":1,"time":1783095399164,"data":{"turn":1,"point":"UserPromptSubmit","dialect":"codex","handlerId":"codex:UserPromptSubmit:1"}}
-{"type":"hook/result","seq":2,"time":1783095399180,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"codex:UserPromptSubmit:1","decision":"pass","exitCode":0}}
+{"type":"hook/result","seq":2,"time":1783095399180,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"codex:UserPromptSubmit:1","decision":"pass","exitCode":0,"durationMs":15.786223999999947}}
 {"type":"user/message","seq":3,"time":1783095399180,"data":{"content":[{"type":"text","text":"What is my favorite color? Reply with just the color and stop. Do not use any tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"context/message","seq":4,"time":1783095399180,"data":{"content":[{"type":"text","text":"The user has previously stated their favorite color is teal."}],"source":{"kind":"plugin","plugin":"hooks-codex"}},"surfaceOp":"append"}
 {"type":"step/start","seq":5,"time":1783095399181,"data":{"turn":1,"step":1}}