feat: context window monitor hook with agent-side WARNING/CRITICAL alerts

Adds PostToolUse hook that reads context metrics from statusline bridge file and injects alerts into agent conversation when context is low.

Features:
- Two-tier alerts: WARNING (<=35% remaining) and CRITICAL (<=25%)
- Smart debounce: 5 tool uses between warnings, severity escalation bypasses
- Silent fail: never blocks tool execution
- Security: session_id sanitized to prevent path traversal

Ref #212

bin/install.js

@@ -892,7 +892,7 @@ function uninstall(isGlobal, runtime = 'claude') {
   // 4. Remove GSD hooks
   const hooksDir = path.join(targetDir, 'hooks');
   if (fs.existsSync(hooksDir)) {
-    const gsdHooks = ['gsd-statusline.js', 'gsd-check-update.js', 'gsd-check-update.sh'];
+    const gsdHooks = ['gsd-statusline.js', 'gsd-check-update.js', 'gsd-check-update.sh', 'gsd-context-monitor.js'];
     let hookCount = 0;
     for (const hook of gsdHooks) {
       const hookPath = path.join(hooksDir, hook);
@@ -958,10 +958,32 @@ function uninstall(isGlobal, runtime = 'claude') {
       if (settings.hooks.SessionStart.length === 0) {
         delete settings.hooks.SessionStart;
       }
-      // Clean up empty hooks object
-      if (Object.keys(settings.hooks).length === 0) {
-        delete settings.hooks;
+    }
+
+    // Remove GSD hooks from PostToolUse
+    if (settings.hooks && settings.hooks.PostToolUse) {
+      const before = settings.hooks.PostToolUse.length;
+      settings.hooks.PostToolUse = settings.hooks.PostToolUse.filter(entry => {
+        if (entry.hooks && Array.isArray(entry.hooks)) {
+          const hasGsdHook = entry.hooks.some(h =>
+            h.command && h.command.includes('gsd-context-monitor')
+          );
+          return !hasGsdHook;
+        }
+        return true;
+      });
+      if (settings.hooks.PostToolUse.length < before) {
+        settingsModified = true;
+        console.log(`  ${green}✓${reset} Removed context monitor hook from settings`);
       }
+      if (settings.hooks.PostToolUse.length === 0) {
+        delete settings.hooks.PostToolUse;
+      }
+    }
+
+    // Clean up empty hooks object
+    if (settings.hooks && Object.keys(settings.hooks).length === 0) {
+      delete settings.hooks;
     }
 
     if (settingsModified) {
@@ -1520,6 +1542,9 @@ function install(isGlobal, runtime = 'claude') {
   const updateCheckCommand = isGlobal
     ? buildHookCommand(targetDir, 'gsd-check-update.js')
     : 'node ' + dirName + '/hooks/gsd-check-update.js';
+  const contextMonitorCommand = isGlobal
+    ? buildHookCommand(targetDir, 'gsd-context-monitor.js')
+    : 'node ' + dirName + '/hooks/gsd-context-monitor.js';
 
   // Enable experimental agents for Gemini CLI (required for custom sub-agents)
   if (isGemini) {
@@ -1556,6 +1581,27 @@ function install(isGlobal, runtime = 'claude') {
       });
       console.log(`  ${green}✓${reset} Configured update check hook`);
     }
+
+    // Configure PostToolUse hook for context window monitoring
+    if (!settings.hooks.PostToolUse) {
+      settings.hooks.PostToolUse = [];
+    }
+
+    const hasContextMonitorHook = settings.hooks.PostToolUse.some(entry =>
+      entry.hooks && entry.hooks.some(h => h.command && h.command.includes('gsd-context-monitor'))
+    );
+
+    if (!hasContextMonitorHook) {
+      settings.hooks.PostToolUse.push({
+        hooks: [
+          {
+            type: 'command',
+            command: contextMonitorCommand
+          }
+        ]
+      });
+      console.log(`  ${green}✓${reset} Configured context window monitor hook`);
+    }
   }
 
   // Write file manifest for future modification detection