fix: resolve JSON escape sequence and tsx dependency issues

Problem matcher fixes:
- Fix JSON escape sequences in .github/matchers/flakeguard.json
- Properly escape square brackets: \[ becomes \[ in JSON regex patterns
- Fixes "Bad JSON escape sequence: \[" error in GitHub Actions problem matcher loading

Dependency fixes:
- Add tsx as devDependency to packages/cli package.json
- Enables pnpm --filter @flakeguard/cli exec tsx commands in CI workflows
- Fixes ERR_PNPM_RECURSIVE_EXEC_FIRST_FAIL Command "tsx" not found

Validation results:
✅ Problem matcher JSON is valid and loads correctly
✅ tsx is accessible via pnpm exec tsx in packages/cli context
✅ FlakeGuard workflows should now complete without dependency errors

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: thc1006

.github/matchers/flakeguard.json

@@ -5,7 +5,7 @@
       "severity": "warning",
       "pattern": [
         {
-          "regexp": "^\[FLAKY\]\s+(.*)\s+in\s+(.*):(\d+)\s+-\s+(.*)$",
+          "regexp": "^\\[FLAKY\\]\\s+(.*)\\ +in\\s+(.*):(\\ +)\\s+-\\s+(.*)$",
           "message": 1,
           "file": 2,
           "line": 3,
@@ -18,7 +18,7 @@
       "severity": "error",
       "pattern": [
         {
-          "regexp": "^\[FAILED\]\s+(.*)\s+in\s+(.*):(\d+)\s+-\s+(.*)$",
+          "regexp": "^\\[FAILED\\]\\s+(.*)\\ +in\\s+(.*):(\\ +)\\s+-\\s+(.*)$",
           "message": 1,
           "file": 2,
           "line": 3,
@@ -31,7 +31,7 @@
       "severity": "notice",
       "pattern": [
         {
-          "regexp": "^\[QUARANTINED\]\s+(.*)\s+in\s+(.*):(\d+)\s+-\s+(.*)$",
+          "regexp": "^\\[QUARANTINED\\]\\s+(.*)\\ +in\\s+(.*):(\\ +)\\s+-\\s+(.*)$",
           "message": 1,
           "file": 2,
           "line": 3,
@@ -40,4 +40,4 @@
       ]
     }
   ]
-}
+}

packages/cli/package.json

@@ -63,6 +63,7 @@
     "@types/pg": "^8.15.5",
     "@types/semver": "^7.7.0",
     "jest": "^29.7.0",
+    "tsx": "^4.20.5",
     "typescript": "^5.9.2"
   },
   "engines": {