Add `mergePullRequest` to the github integration client (let agents merge without `gh`)

[khaliqgant]

Summary

The github integration client (GithubClient) can createPullRequest, postReview, comment, etc. — but has no merge method. So an agent that wants to merge a PR has to shell out to gh pr merge, which (a) is the lone gh/git use left in the showcase agents, and (b) doesn't fit the sandbox model (the repo is a tarball mount with no .git/gh auth; GitHub writes are meant to go through the integration's writeback).

Add mergePullRequest(...) so agents merge through the integration like every other GitHub write.

Not blocking

The review agent (AgentWorkforce/agents#1, review/agent.ts → mergePr) works today via gh pr merge. This is a cleanup that removes the last gh call; nothing depends on it.

Proposed API

// GithubClient
mergePullRequest(args: {
 owner: string;
 repo: string;
 number: number;
 method?: 'merge' | 'squash' | 'rebase'; // default 'squash'
 commitTitle?: string;
 commitMessage?: string;
}): Promise<{ merged: boolean; sha?: string }>;

Usage in the review agent then becomes:

await ctx.github.mergePullRequest({ owner, repo, number, method: 'squash' });

Repos / files to touch

Mirror the createPullRequest path (client writes a VFS draft → writeback worker calls the GitHub REST API):

1. AgentWorkforce/workforce — packages/runtime/src/clients/github.ts
   Add mergePullRequest to the GithubClient interface + impl (write a draft via writeJsonFile, e.g. to /github/repos/{owner}/{repo}/pulls/{n}/merge.json).
2. AgentWorkforce/relayfile-adapters — packages/github/src/writeback.ts (GitHubWritebackHandler, today only handles pulls/{n}/reviews/...) + packages/github/github.mapping.yaml (writebacks: block)
   Map the merge draft path → PUT /repos/{owner}/{repo}/pulls/{pull_number}/merge (body: merge_method, optional commit_title/commit_message). Add a writeback.test.ts case.
3. AgentWorkforce/cloud — verify the relayfile-writeback-consumer executes the new request shape (likely generic — no change, just confirm).
4. AgentWorkforce/agents — review/agent.ts mergePr: swap ctx.sandbox.exec('gh pr merge …') for ctx.github.mergePullRequest(...) (drops the last gh).

Acceptance

• ctx.github.mergePullRequest(...) merges a PR end-to-end via the writeback (no gh).
• review agent merges on authorized approval with no gh/sandbox shell-out.

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