feat(mcp): add octobroker_commit_status_set broker-owned tool

[chaodu-agent]

Summary

Review agents need to publish a PR-review verdict as a commit status check (e.g. context: "OpenAB PR Review", state: failure), but the upstream GitHub MCP server offers no commit-status mutation at all — only the read-only get_status. This surfaced in live operation: a review bot correctly refused to fabricate an endpoint and reported the capability gap.

This PR adds a second octobroker-owned local tool, octobroker_commit_status_set, following the octobroker_review_minimize_comment pattern from #45 (issue #44).

Changes

• LOCAL_TOOLS registry — the early write gate, local dispatch, tools/list injection, and x-mcp-tools upstream filtering are generalized over all broker-owned tools. Each tool still requires its own explicit per-agent allowlist entry to be advertised or callable.
• handle_commit_status_set — executes POST /repos/{owner}/{repo}/statuses/{sha} locally through the scoped GitHub App credential. Never forwarded upstream.
• Strict validation — owner/repo/sha become URL path segments: charset-checked and full 40/64-hex SHA enforced, so arguments cannot escape the policy-checked repository path. state must be one of error|failure|pending|success; context non-empty; description ≤ 140 chars; target_url must be http(s).
• Fail closed — only HTTP 201 with an echoed state counts as success; anything else returns a tool error. Phase 1 (no-agent) mode and writes-disabled agents are denied before credential resolution.
• Same guardrails as upstream writes — repository allowlist, write gate, per-agent in-flight cap, and fail-closed JSONL audit (pre-flight + result) all apply unchanged.
• README — tool documentation + Commit statuses: write row in the App permission table.

Tests

8 new/updated tests; full suite 132 passed, 0 failed, clippy clean:

• success path posts the exact /repos/{owner}/{repo}/statuses/{sha} path and payload (mock REST server)
• bad arguments (state enum, short SHA, path-escape owner/repo/sha, blank context, javascript: target_url, >140-char description) rejected before any network call
• GitHub 422 and non-201 responses fail closed
• Phase 1 mode denies the tool (403, nothing reaches upstream)
• tools/list injection requires each local tool's own allowlist entry; x-mcp-tools filter strips all local tools

App permission

Requires Commit statuses: write on the GitHub App installation(s) — grant + per-installation approval needed before agents can use it.