feat(hooks): auto-record session state on context exhaustion (#1974)

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>

hooks/gsd-context-monitor.js

@@ -128,6 +128,21 @@ process.stdin.on('end', () => {
     // Detect if GSD is active (has .planning/STATE.md in working directory)
     const isGsdActive = fs.existsSync(path.join(cwd, '.planning', 'STATE.md'));
 
+    // On CRITICAL with active GSD project, auto-record session state as a
+    // breadcrumb for /gsd-resume-work (#1974). Fire-and-forget subprocess —
+    // doesn't block the hook or the agent.
+    if (isCritical && isGsdActive) {
+      try {
+        const gsdTools = path.join(process.env.HOME || '', '.claude', 'get-shit-done', 'bin', 'gsd-tools.cjs');
+        const timestamp = new Date().toISOString().split('T')[0];
+        require('child_process').spawn(
+          process.execPath,
+          [gsdTools, 'state', 'update', 'Stopped At', `context exhaustion at ${usedPct}% (${timestamp})`],
+          { cwd, detached: true, stdio: 'ignore' }
+        ).unref();
+      } catch { /* non-critical — don't let state recording break the hook */ }
+    }
+
     // Build advisory warning message (never use imperative commands that
     // override user preferences — see #884)
     let message;