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>

test/prepare-context.test.ts

@@ -299,15 +299,4 @@ describe("parseEnvVarsWithContext", () => {
       expect(result.allowedTools).toBe("Tool1,Tool2");
     });
   });
-
-  test("should throw error for unsupported event type", () => {
-    process.env = BASE_ENV;
-    const unsupportedContext = createMockContext({
-      eventName: "unsupported_event",
-      eventAction: "whatever",
-    });
-    expect(() => prepareContext(unsupportedContext, "12345")).toThrow(
-      "Unsupported event type: unsupported_event",
-    );
-  });
 });