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/modes/registry.test.ts

@@ -3,18 +3,18 @@ import { getMode, isValidMode } from "../../src/modes/registry";
 import type { ModeName } from "../../src/modes/types";
 import { tagMode } from "../../src/modes/tag";
 import { agentMode } from "../../src/modes/agent";
-import { createMockContext } from "../mockContext";
+import { createMockContext, createMockAutomationContext } from "../mockContext";
 
 describe("Mode Registry", () => {
   const mockContext = createMockContext({
     eventName: "issue_comment",
   });
 
-  const mockWorkflowDispatchContext = createMockContext({
+  const mockWorkflowDispatchContext = createMockAutomationContext({
     eventName: "workflow_dispatch",
   });
 
-  const mockScheduleContext = createMockContext({
+  const mockScheduleContext = createMockAutomationContext({
     eventName: "schedule",
   });