refactor: implement discriminated unions for GitHub contexts (#360)

* 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

* add schedule & workflow dispatch paths. Also make prepare logic conditional

* tests

* Add test workflow for workflow_dispatch functionality

* Update workflow to use correct branch reference

* remove test workflow dispatch file

* minor lint update

* update workflow dispatch agent example

* minor lint update

* refactor: simplify prepare logic with mode-specific implementations

* ensure tag mode can't work with workflow dispatch and schedule tasks

* simplify: remove workflow_dispatch/schedule from create-prompt

- Remove workflow_dispatch and schedule event handling from create-prompt
  since agent mode doesn't use the standard prompt generation flow
- Enforce mode compatibility at selection time in the registry instead
  of runtime validation in tag mode
- Add explanatory comment in agent mode about why prompt file is needed
- Update tests to reflect simplified event handling

This reduces code duplication and makes the separation between tag mode
(entity-based events) and agent mode (automation events) clearer.

* simplify PR by making agent mode only work with workflow dispatch and schedule events

* remove unnecessary changes

* remove unnecessary changes from PR

- Revert update-comment-link.ts changes (agent mode doesn't use this)
- Revert create-initial.ts changes (agent mode doesn't create comments)
- Remove unused default-branch.ts file
- Revert install-mcp-server.ts changes (agent mode uses minimal MCP)

These files are only used by tag mode for entity-based events, not needed
for workflow_dispatch/schedule support via agent mode.

* fix: handle optional entityNumber for TypeScript

- Add runtime checks in files that require entityNumber
- These files are only used by tag mode which always has entityNumber
- Agent mode (workflow_dispatch/schedule) doesn't use these files

* linting update

* refactor: implement discriminated unions for GitHub contexts

Split ParsedGitHubContext into entity-specific and automation contexts:
- ParsedGitHubContext: For entity events (issues/PRs) with required entityNumber and isPR
- AutomationContext: For workflow_dispatch/schedule events without entity fields
- GitHubContext: Union type for all contexts

This eliminates ~20 null checks throughout the codebase and provides better type safety.
Entity-specific code paths are now guaranteed to have the required fields.

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

* update comment

* More robust type checking

* refactor: improve discriminated union implementation based on review feedback

- Use eventName checks instead of 'in' operator for more robust type guards
- Remove unnecessary type assertions - TypeScript's control flow analysis works correctly
- Remove redundant runtime checks for entityNumber and isPR
- Simplify code by using context directly after type guard

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

* some structural simplification

* refactor: further simplify discriminated union implementation

- Add event name constants to reduce duplication
- Derive EntityEventName and AutomationEventName types from constants
- Use isAutomationContext consistently in agent mode and registry
- Simplify parseGitHubContext by removing redundant type assertions
- Extract payload casts to variables for cleaner code

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

* bun format

* specify the type

* minor linting update again

---------

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

src/entrypoints/prepare.ts

@@ -9,7 +9,7 @@ import * as core from "@actions/core";
 import { setupGitHubToken } from "../github/token";
 import { checkWritePermissions } from "../github/validation/permissions";
 import { createOctokit } from "../github/api/client";
-import { parseGitHubContext } from "../github/context";
+import { parseGitHubContext, isEntityContext } from "../github/context";
 import { getMode } from "../modes/registry";
 import { prepare } from "../prepare";
 
@@ -22,15 +22,17 @@ async function run() {
     // Step 2: Parse GitHub context (once for all operations)
     const context = parseGitHubContext();
 
-    // Step 3: Check write permissions
-    const hasWritePermissions = await checkWritePermissions(
-      octokit.rest,
-      context,
-    );
-    if (!hasWritePermissions) {
-      throw new Error(
-        "Actor does not have write permissions to the repository",
+    // Step 3: Check write permissions (only for entity contexts)
+    if (isEntityContext(context)) {
+      const hasWritePermissions = await checkWritePermissions(
+        octokit.rest,
+        context,
       );
+      if (!hasWritePermissions) {
+        throw new Error(
+          "Actor does not have write permissions to the repository",
+        );
+      }
     }
 
     // Step 4: Get mode and check trigger conditions