When the context monitor detects CRITICAL threshold (25% remaining)
and a GSD project is active, spawn a fire-and-forget subprocess to
record "Stopped At: context exhaustion at N%" in STATE.md.
This provides automatic breadcrumbs for /gsd-resume-work when sessions
crash from context exhaustion — the most common unrecoverable scenario.
Previously, session state was only saved via voluntary /gsd-pause-work.
The subprocess is detached and unref'd so it doesn't block the hook
or the agent. The advisory warning to the agent is unchanged.
Closes#1974
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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