test: sidebar agent test suite (layers 1-2)

Layer 1 (unit): 18 tests for URL sanitization in sidebar-utils.ts — http/https
pass, chrome:// rejected, javascript: rejected, control chars stripped, truncation.

Layer 2 (integration): 13 tests for server HTTP endpoints — auth, sidebar-command
queue writes, activeTabUrl override/fallback, event relay to chat buffer, message
queuing, queue overflow (429), chat clear, agent kill.

Source changes for testability:
- Extract sanitizeExtensionUrl() to browse/src/sidebar-utils.ts
- Add BROWSE_HEADLESS_SKIP env var to skip browser launch in HTTP-only tests
- Add SIDEBAR_QUEUE_PATH env var to both server.ts and sidebar-agent.ts
- Add SIDEBAR_AGENT_TIMEOUT env var to sidebar-agent.ts
- Sync package.json version to match VERSION (0.12.2.0)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "0.12.0.0",
+  "version": "0.12.2.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",