✨ feat: merge PRs and native stacks via async API

[SigureMo]

Motivation

Native stack merge hints currently open a browser, while ordinary PRs delegate to gh pr merge, which still uses the synchronous GraphQL mutation. Add gh-llm pr merge backed by GitHub's async merge API, so both workflows stay in the terminal without requiring gh-stack.

The command pins the inspected PR head, shows the selected stack scope, and polls for merged, enqueued, or failed. Pending requests retain a UUID and a pr merge-status resume command; exit code 2 signals that the request is still pending. Existing requests are followed with their original options, merge writes are not automatically retried, and entering a merge queue is reported separately from merging. Direct merge methods, commit messages, queue selection, and explicit rule bypass are supported.

Replace merge hints with head-pinned commands and update the README and conversation skill. Incomplete stack metadata continues to suppress merge suggestions.

Validation

• uv run pytest -q: 155 passed, including API contract fixtures for duplicate requests, pending timeouts, resume polling, transport failures, stale heads, and stack scope.
• uv run ty check --error-on-warning src/gh_llm tests, Ruff lint/format, Prettier, and git diff --check passed.
• Checked both command help screens. Merge behavior was tested with API response fixtures; no live PR was merged.

^{This PR is co-authored with @codex (gpt-6 astra xhigh)}

[chatgpt-codex-connector]

Summary

• Corrected async merge requests to send GitHub’s expected_head_sha parameter instead of the synchronous merge API’s sha parameter, ensuring merges are pinned to the inspected PR head. src/gh_llm/github_api.pyL1731-L1742
• Updated the API contract test to verify the correct head-precondition field. tests/test_pr_merge.pyL79-L84
• Committed the change as c585e29 and created the follow-up PR titled “fix: pin async merge requests to the inspected head.”

Testing

• ✅ ruff check .
• ✅ ruff format --check .
• ✅ prettier --check README.md skills/github-conversation/SKILL.md
• ✅ git diff --check
• ✅ PYENV_VERSION=3.14.4 PYTHONPATH=src python -m compileall -q src tests
• ⚠️ PYENV_VERSION=3.14.4 uv run pytest -q (environment could not download the uncached packaging==24.1 dependency because outbound package access was unavailable)
• ⚠️ ty check --error-on-warning src/gh_llm tests (ty is not installed globally, and the UV environment could not be provisioned due to the dependency-download limitation)

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