fix: add checkHumanActor to agent mode (#826)

Fixes issue #641 where users were getting banned due to rapid successive
Claude runs triggered by the synchronize event.

Changes:
- Add checkHumanActor call to agent mode's prepare() method to reject
  bot-triggered workflows unless explicitly allowed via allowed_bots
- Update checkHumanActor to accept GitHubContext (union type) instead
  of just ParsedGitHubContext
- Add tests for bot rejection/allowance in agent mode

Claude-Generated-By: Claude Code (cli/claude-opus-4-5=100%)
Claude-Steers: 1
Claude-Permission-Prompts: 3
Claude-Escapes: 0

src/github/validation/actor.ts

@@ -6,11 +6,11 @@
  */
 
 import type { Octokit } from "@octokit/rest";
-import type { ParsedGitHubContext } from "../context";
+import type { GitHubContext } from "../context";
 
 export async function checkHumanActor(
   octokit: Octokit,
-  githubContext: ParsedGitHubContext,
+  githubContext: GitHubContext,
 ) {
   // Fetch user information from GitHub API
   const { data: userData } = await octokit.users.getByUsername({