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

scripts/build-hooks.js

@@ -12,6 +12,7 @@ const DIST_DIR = path.join(HOOKS_DIR, 'dist');
 // Hooks to copy (pure Node.js, no bundling needed)
 const HOOKS_TO_COPY = [
   'gsd-check-update.js',
+  'gsd-context-monitor.js',
   'gsd-statusline.js'
 ];