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>

src/github/operations/comments/create-initial.ts

@@ -22,12 +22,6 @@ export async function createInitialComment(
 ) {
   const { owner, repo } = context.repository;
 
-  // This function is only called for entity-based events
-  if (!context.entityNumber) {
-    throw new Error("createInitialComment requires an entity number");
-  }
-  const entityNumber = context.entityNumber;
-
   const jobRunLink = createJobRunLink(owner, repo, context.runId);
   const initialBody = createCommentBody(jobRunLink);
 
@@ -42,7 +36,7 @@ export async function createInitialComment(
       const comments = await octokit.rest.issues.listComments({
         owner,
         repo,
-        issue_number: entityNumber,
+        issue_number: context.entityNumber,
       });
       const existingComment = comments.data.find((comment) => {
         const idMatch = comment.user?.id === CLAUDE_APP_BOT_ID;
@@ -65,7 +59,7 @@ export async function createInitialComment(
         response = await octokit.rest.issues.createComment({
           owner,
           repo,
-          issue_number: entityNumber,
+          issue_number: context.entityNumber,
           body: initialBody,
         });
       }
@@ -74,7 +68,7 @@ export async function createInitialComment(
       response = await octokit.rest.pulls.createReplyForReviewComment({
         owner,
         repo,
-        pull_number: entityNumber,
+        pull_number: context.entityNumber,
         comment_id: context.payload.comment.id,
         body: initialBody,
       });
@@ -83,7 +77,7 @@ export async function createInitialComment(
       response = await octokit.rest.issues.createComment({
         owner,
         repo,
-        issue_number: entityNumber,
+        issue_number: context.entityNumber,
         body: initialBody,
       });
     }
@@ -101,7 +95,7 @@ export async function createInitialComment(
       const response = await octokit.rest.issues.createComment({
         owner,
         repo,
-        issue_number: entityNumber,
+        issue_number: context.entityNumber,
         body: initialBody,
       });