fix(security): CSS injection guard, timeout clamping, session validation, tests (#806)

Community PR #806 by @mr-k-man (security audit round 2, new parts only).

- CSS value validation (DANGEROUS_CSS) in cdp-inspector, write-commands, extension inspector
- Queue file permissions (0o700/0o600) in cli, server, sidebar-agent
- escapeRegExp for frame --url ReDoS fix
- Responsive screenshot path validation with validateOutputPath
- State load cookie filtering (reject localhost/.internal/metadata cookies)
- Session ID format validation in loadSession
- /health endpoint: remove currentUrl and currentMessage fields
- QueueEntry interface + isValidQueueEntry validator for sidebar-agent
- SIGTERM->SIGKILL escalation in timeout handler
- Viewport dimension clamping (1-16384), wait timeout clamping (1s-300s)
- Cookie domain validation in cookie-import and cookie-import-browser
- DocumentFragment-based tab switching (XSS fix in sidepanel)
- pollInProgress reentrancy guard for pollChat
- toggleClass/injectCSS input validation in extension inspector
- Snapshot annotated path validation with realpathSync
- 714-line security-audit-r2.test.ts + 33-line learnings-injection.test.ts

Co-Authored-By: mr-k-man <mr-k-man@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/inspector.js

@@ -355,6 +355,10 @@
   function applyStyle(selector, property, value) {
     // Validate property name: alphanumeric + hyphens only
     if (!/^[a-zA-Z-]+$/.test(property)) return { error: 'Invalid property name' };
+    // Validate CSS value: block exfiltration vectors (url(), expression(), @import, javascript:, data:)
+    if (/url\s*\(|expression\s*\(|@import|javascript:|data:/i.test(value)) {
+      return { error: 'CSS value contains blocked pattern' };
+    }
 
     const el = findElement(selector);
     if (!el) return { error: 'Element not found' };
@@ -373,6 +377,9 @@
   }
 
   function toggleClass(selector, className, action) {
+    if (!/^[a-zA-Z0-9_-]+$/.test(className)) {
+      return { error: 'Invalid class name' };
+    }
     const el = findElement(selector);
     if (!el) return { error: 'Element not found' };
 
@@ -387,6 +394,12 @@
   }
 
   function injectCSS(id, css) {
+    if (!/^[a-zA-Z0-9_-]+$/.test(id)) {
+      return { error: 'Invalid CSS injection id' };
+    }
+    if (/url\s*\(|expression\s*\(|@import|javascript:|data:/i.test(css)) {
+      return { error: 'CSS contains blocked pattern (url, expression, @import)' };
+    }
     const styleId = `gstack-inject-${id}`;
     let styleEl = document.getElementById(styleId);
     if (!styleEl) {