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/mockContext.ts

@@ -1,4 +1,7 @@
-import type { ParsedGitHubContext } from "../src/github/context";
+import type {
+  ParsedGitHubContext,
+  AutomationContext,
+} from "../src/github/context";
 import type {
   IssuesEvent,
   IssueCommentEvent,
@@ -38,7 +41,7 @@ export const createMockContext = (
 ): ParsedGitHubContext => {
   const baseContext: ParsedGitHubContext = {
     runId: "1234567890",
-    eventName: "",
+    eventName: "issue_comment", // Default to a valid entity event
     eventAction: "",
     repository: defaultRepository,
     actor: "test-actor",
@@ -55,6 +58,22 @@ export const createMockContext = (
   return { ...baseContext, ...overrides };
 };
 
+export const createMockAutomationContext = (
+  overrides: Partial<AutomationContext> = {},
+): AutomationContext => {
+  const baseContext: AutomationContext = {
+    runId: "1234567890",
+    eventName: "workflow_dispatch",
+    eventAction: undefined,
+    repository: defaultRepository,
+    actor: "test-actor",
+    payload: {} as any,
+    inputs: defaultInputs,
+  };
+
+  return { ...baseContext, ...overrides };
+};
+
 export const mockIssueOpenedContext: ParsedGitHubContext = {
   runId: "1234567890",
   eventName: "issues",