fix: match Codex sessions by session_meta timestamp not file birthtime

[mvanhorn]

Summary

CodexAdapter now matches a running Codex process to its session by the logical session start time recorded in session_meta.payload.timestamp, rather than the session file's filesystem birth time.

Codex registers a session before it writes the session JSONL file. If the agent sits idle at the prompt longer than the matching tolerance (abs(process.startTime - session.birthtimeMs) <= 3 min in matchProcessesToSessions), the file is created late, its birthtimeMs drifts past the process start time, and the session never pairs with its PID. Transcript polling, conversation detail, and wait mode all break because sessionFilePath is never resolved.

The fix prefers the session_meta payload timestamp as the birthtimeMs used for matching in both code paths that build a SessionFile:

• discoverSessions (bulk enumeration), where the first session_meta line was already parsed to set resolvedCwd.
• findSessionFileById (the codex resume <uuid> path).

When the metadata timestamp is missing or unparseable, both paths fall back to the filesystem stat.birthtimeMs, so existing behavior is unchanged. matching.ts is untouched: it still compares process.startTime against session.birthtimeMs, which now carries the true session start when available.

Why this matters

Closes the gap described in #105: a Codex agent left idle for more than three minutes before its first message would silently fail to surface its transcript, conversation, and wait status, because the late-written file's birth time fell outside the matching window even though the process and session were the same.

Testing

• Added CodexAdapter cases covering the aligned timestamp/birthtime happy path, the late-created-file recovery via metadata timestamp, fallback to filesystem birthtime when the timestamp is missing or invalid, and malformed/unreadable inputs.
• vitest run src/__tests__/adapters/CodexAdapter.test.ts in packages/agent-manager: 59 passed.
• tsc --noEmit and eslint clean on the touched files.

Fixes #105

[codeaholicguy]

LGTM! Thanks for the contribution.

[mvanhorn]

Thanks @codeaholicguy for the merge. Matching Codex sessions by session_meta timestamp is more robust than the old approach.