Use the Codex session file's mtime as a proxy for agent activity.
Codex continuously appends to its rollout JSONL while working, so:
- recently modified file → active
- stale file (past threshold) → idle
- process not running → exited
This replaces the previous null return with real activity detection,
using the same threshold constant (DEFAULT_READY_THRESHOLD_MS = 5min)
as the Claude Code plugin.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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