Add merge-pr safety preflight skill

[andrei-hasna]

Why

PR #215 contains the desired merge-pr skill work, but it targets fleet/spark02-preserve-20260625-codewith. Retargeting that PR to main would include unrelated preservation-branch changes, which makes it unsuitable as a releaseable mainline PR.

This PR supersedes #215 for releaseability by starting from current main and carrying only .codewith/skills/merge-pr/** from de0db42c005d80b5d1ec900023311af53a645730.

What changed

• Adds the Codewith merge-pr skill instructions and merge safety reference.
• Adds a read-only merge_pr_preflight.py helper for PR mergeability snapshots.
• Supports current and legacy gh pr checks status fields such as bucket, state, status, and conclusion.
• Adds fixture-backed tests for successful, failed, pending, and actual-merge artifact-blocking preflight cases.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 .codewith/skills/merge-pr/tests/test_merge_pr_preflight.py
• python3 -m py_compile .codewith/skills/merge-pr/scripts/merge_pr_preflight.py
• git diff --check
• git diff --cached --check
• shield review
• todos redaction scan --file over each staged file
• python3 .codewith/skills/merge-pr/scripts/merge_pr_preflight.py 215 --repo hasna/codewith --mode preflight

The live PR #215 preflight was read-only and confirmed the current gh pr checks JSON shape is parsed. It reported #215 as not_mergeable because that preservation-branch PR currently has failing and pending checks.

[github-actions]

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