fix: use original title from webhook payload instead of fetched title (#793)

* fix: use original title from webhook payload instead of fetched title

- Add extractOriginalTitle() helper to extract title from webhook payload
- Add originalTitle parameter to fetchGitHubData()
- Update tag mode to pass original title from webhook context
- Add tests for extractOriginalTitle and originalTitle parameter

This ensures the title used in prompts is the one that existed when the
trigger event occurred, rather than a potentially modified title fetched
later via GraphQL.

* fix: add title sanitization and explicit TOCTOU test

- Apply sanitizeContent() to titles in formatContext() for defense-in-depth
- Add explicit test documenting TOCTOU prevention for title handling

test/data-fetcher.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it, jest } from "bun:test";
 import {
   extractTriggerTimestamp,
+  extractOriginalTitle,
   fetchGitHubData,
   filterCommentsToTriggerTime,
   filterReviewsToTriggerTime,
@@ -9,6 +10,7 @@ import {
 import {
   createMockContext,
   mockIssueCommentContext,
+  mockPullRequestCommentContext,
   mockPullRequestReviewContext,
   mockPullRequestReviewCommentContext,
   mockPullRequestOpenedContext,
@@ -63,6 +65,47 @@ describe("extractTriggerTimestamp", () => {
   });
 });
 
+describe("extractOriginalTitle", () => {
+  it("should extract title from IssueCommentEvent on PR", () => {
+    const title = extractOriginalTitle(mockPullRequestCommentContext);
+    expect(title).toBe("Fix: Memory leak in user service");
+  });
+
+  it("should extract title from PullRequestReviewEvent", () => {
+    const title = extractOriginalTitle(mockPullRequestReviewContext);
+    expect(title).toBe("Refactor: Improve error handling in API layer");
+  });
+
+  it("should extract title from PullRequestReviewCommentEvent", () => {
+    const title = extractOriginalTitle(mockPullRequestReviewCommentContext);
+    expect(title).toBe("Performance: Optimize search algorithm");
+  });
+
+  it("should extract title from pull_request event", () => {
+    const title = extractOriginalTitle(mockPullRequestOpenedContext);
+    expect(title).toBe("Feature: Add user authentication");
+  });
+
+  it("should extract title from issues event", () => {
+    const title = extractOriginalTitle(mockIssueOpenedContext);
+    expect(title).toBe("Bug: Application crashes on startup");
+  });
+
+  it("should return undefined for event without title", () => {
+    const context = createMockContext({
+      eventName: "issue_comment",
+      payload: {
+        comment: {
+          id: 123,
+          body: "test",
+        },
+      } as any,
+    });
+    const title = extractOriginalTitle(context);
+    expect(title).toBeUndefined();
+  });
+});
+
 describe("filterCommentsToTriggerTime", () => {
   const createMockComment = (
     createdAt: string,
@@ -945,4 +988,115 @@ describe("fetchGitHubData integration with time filtering", () => {
     );
     expect(hasPrBodyInMap).toBe(false);
   });
+
+  it("should use originalTitle when provided instead of fetched title", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Fetched Title From GraphQL",
+            body: "PR body",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+      originalTitle: "Original Title From Webhook",
+    });
+
+    expect(result.contextData.title).toBe("Original Title From Webhook");
+  });
+
+  it("should use fetched title when originalTitle is not provided", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Fetched Title From GraphQL",
+            body: "PR body",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+    });
+
+    expect(result.contextData.title).toBe("Fetched Title From GraphQL");
+  });
+
+  it("should use original title from webhook even if title was edited after trigger", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Edited Title (from GraphQL)",
+            body: "PR body",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            lastEditedAt: "2024-01-15T12:30:00Z", // Edited after trigger
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+      triggerTime: "2024-01-15T12:00:00Z",
+      originalTitle: "Original Title (from webhook at trigger time)",
+    });
+
+    expect(result.contextData.title).toBe(
+      "Original Title (from webhook at trigger time)",
+    );
+  });
 });