feat(KB-052): refactor GitHub integration to use gh CLI

- Create gh-cli utility module with auth detection and command execution
- Refactor GitHubClient to use gh CLI commands with REST API fallback
- Refactor PR Monitor to use gh CLI for PR operations
- Add listIssues() and getIssue() methods to GitHubClient
- Remove in-app GitHubRateLimiter (gh CLI handles rate limiting)
- Update all tests for gh CLI implementation
- Update AGENTS.md and extension docs for gh CLI auth preference

packages/core/package.json

@@ -6,6 +6,10 @@
     ".": {
       "import": "./dist/index.js",
       "types": "./dist/index.d.ts"
+    },
+    "./gh-cli": {
+      "import": "./dist/gh-cli.js",
+      "types": "./dist/gh-cli.d.ts"
     }
   },
   "publishConfig": {