fix(hooks): address three blocking defects in context exhaustion record (#1974)

Review feedback from @trek-e — three blocking fixes:

1. **Sentinel prevents repeated firing**
   Added warnData.criticalRecorded flag persisted to the warn state file.
   Previously the subprocess fired on every DEBOUNCE_CALLS cycle (5 tool
   uses) for the rest of the session, overwriting the "crash moment"
   record with a new timestamp each time. Now fires exactly once per
   CRITICAL session.

2. **Runtime-agnostic path via __dirname**
   Replaced hardcoded `path.join(process.env.HOME, '.claude', ...)` with
   `path.join(__dirname, '..', 'get-shit-done', 'bin', 'gsd-tools.cjs')`.
   The hook lives at <runtime-config>/hooks/ and gsd-tools.cjs at
   <runtime-config>/get-shit-done/bin/ — __dirname resolves correctly on
   all runtimes (Claude Code, OpenCode, Gemini, Kilo) without assuming
   ~/.claude/.

3. **Correct subcommand: state record-session**
   Switched from `state update "Stopped At" ...` to
   `state record-session --stopped-at ...`. The dedicated command
   updates Last session, Last Date, Stopped At, and Resume File
   atomically under the state lock.

Also:
- Hoisted `const { spawn } = require('child_process')` to top of file
  to match existing require() style.
- Coerced usedPct to Number(usedPct) || 0 to sanitize the bridge file
  in case it's malformed or adversarially crafted.

Tests (tests/bug-1974-context-exhaustion-record.test.cjs, 4 cases):
- Subprocess spawns and writes "context exhaustion" on CRITICAL
- Subprocess does NOT spawn when .planning/STATE.md is absent
- Sentinel guard prevents second fire within same session
- Hook source uses __dirname-based path (not hardcoded ~/.claude/)

hooks/gsd-context-monitor.js

@@ -21,6 +21,7 @@
 const fs = require('fs');
 const os = require('os');
 const path = require('path');
+const { spawn } = require('child_process');
 
 const WARNING_THRESHOLD = 35;  // remaining_percentage <= 35%
 const CRITICAL_THRESHOLD = 25; // remaining_percentage <= 25%
@@ -130,16 +131,27 @@ process.stdin.on('end', () => {
 
     // 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) {
+    // doesn't block the hook or the agent. Fires ONCE per CRITICAL session,
+    // guarded by warnData.criticalRecorded to prevent repeated overwrites
+    // of the "crash moment" record on every debounce cycle.
+    if (isCritical && isGsdActive && !warnData.criticalRecorded) {
       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(
+        // Runtime-agnostic path: this hook lives at <runtime-config>/hooks/
+        // and gsd-tools.cjs lives at <runtime-config>/get-shit-done/bin/.
+        // Using __dirname makes this work on Claude Code, OpenCode, Gemini,
+        // Kilo, etc. without hardcoding ~/.claude/.
+        const gsdTools = path.join(__dirname, '..', 'get-shit-done', 'bin', 'gsd-tools.cjs');
+        // Coerce usedPct to a safe number in case bridge file is malformed
+        const safeUsedPct = Number(usedPct) || 0;
+        const stoppedAt = `context exhaustion at ${safeUsedPct}% (${new Date().toISOString().split('T')[0]})`;
+        spawn(
           process.execPath,
-          [gsdTools, 'state', 'update', 'Stopped At', `context exhaustion at ${usedPct}% (${timestamp})`],
+          [gsdTools, 'state', 'record-session', '--stopped-at', stoppedAt],
           { cwd, detached: true, stdio: 'ignore' }
         ).unref();
+        warnData.criticalRecorded = true;
+        // Persist the sentinel so subsequent debounce cycles don't re-fire
+        fs.writeFileSync(warnPath, JSON.stringify(warnData));
       } catch { /* non-critical — don't let state recording break the hook */ }
     }