Derive trigger timestamps for issues and pull_request events (#1592)

* Derive trigger timestamps for issues and pull_request events

For issues labeled/assigned triggers, look up the matching event in the
issue's event history to get the exact time of the label/assignment,
falling back to the payload's updated_at/created_at when the lookup
fails. issues opened uses issue.created_at; pull_request opened uses
pull_request.created_at and other pull_request actions use updated_at.

* Ignore issue label/assign events older than the payload snapshot

A matching labeled/assigned event that predates the webhook payload's
issue.updated_at cannot be the event that fired the webhook, so fall
back to the payload timestamps instead of adopting it as the boundary.

test/mockContext.ts

@@ -140,7 +140,7 @@ export const mockIssueOpenedContext: ParsedGitHubContext = {
       body: "## Description\n\nThe application crashes immediately after launching.\n\n## Steps to reproduce\n\n1. Install the app\n2. Launch it\n3. See crash\n\n/claude please help me fix this",
       assignee: null,
       created_at: "2024-01-15T10:30:00Z",
-      updated_at: "2024-01-15T10:30:00Z",
+      updated_at: "2024-01-15T10:35:00Z",
       html_url: "https://github.com/test-owner/test-repo/issues/42",
       user: {
         login: "john-doe",
@@ -191,6 +191,8 @@ export const mockIssueAssignedContext: ParsedGitHubContext = {
         avatar_url: "https://avatars.githubusercontent.com/u/11111",
         html_url: "https://github.com/claude-bot",
       },
+      created_at: "2024-01-15T09:00:00Z",
+      updated_at: "2024-01-15T11:00:00Z",
     },
     repository: {
       name: "test-repo",
@@ -225,6 +227,8 @@ export const mockIssueLabeledContext: ParsedGitHubContext = {
         html_url: "https://github.com/alice-wonder",
       },
       assignee: null,
+      created_at: "2024-01-15T09:30:00Z",
+      updated_at: "2024-01-15T11:30:00Z",
     },
     label: {
       id: 987654321,
@@ -355,6 +359,8 @@ export const mockPullRequestOpenedContext: ParsedGitHubContext = {
         avatar_url: "https://avatars.githubusercontent.com/u/55555",
         html_url: "https://github.com/feature-developer",
       },
+      created_at: "2024-01-15T14:00:00Z",
+      updated_at: "2024-01-15T14:05:00Z",
     },
     repository: {
       name: "test-repo",