feat: add agent mode for automation scenarios (#337)

* feat: add agent mode for automation scenarios

- Add agent mode that always triggers without checking for mentions
- Implement Mode interface with support for mode-specific tool configuration
- Add getAllowedTools() and getDisallowedTools() methods to Mode interface
- Simplify tests by combining related test cases
- Update documentation and examples to include agent mode
- Fix TypeScript imports to prevent circular dependencies

Agent mode is designed for automation and workflow_dispatch scenarios
where Claude should always run without requiring trigger phrases.

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

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

* Minor update to readme (from @main to @beta)

* Since workflow_dispatch isn't in the base action, update the examples accordingly

* minor formatting issue

* Update to say beta instead of main

* Fix missed tracking comment to be false

---------

Co-authored-by: km-anthropic <km-anthropic@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

src/entrypoints/prepare.ts

@@ -55,7 +55,7 @@ async function run() {
     await checkHumanActor(octokit.rest, context);
 
     // Step 6: Create initial tracking comment (mode-aware)
-    // Some modes (e.g., future review/freeform modes) may not need tracking comments
+    // Some modes (e.g., agent mode) may not need tracking comments
     let commentId: number | undefined;
     let commentData:
       | Awaited<ReturnType<typeof createInitialComment>>