feat: add purge command and web GUI

- Add `purgeTasks` function to tasks.ts with dry-run support
- Add `task-tracker purge` CLI command with --dry-run and --json flags
- Add `src/scanner.ts` for scanning root + immediate subdirs for .tasks.jsonl
- Add `src/gui.ts` HTTP server with full CRUD API (no new runtime deps)
- Add `public/index.html` SPA frontend with Tailwind CDN, stage gauge, timezone selector, auto-refresh
- Export purgeTasks and scanTaskFiles from index.ts
- Add public/ to package.json files array for npm publish
- Add unit tests: purge.test.ts, scanner.test.ts, CLI integration tests for purge

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: metyatech

package.json

@@ -8,6 +8,7 @@
   },
   "files": [
     "dist",
+    "public",
     "LICENSE",
     "README.md"
   ],