Surface repo settings drift in snapshots

[cbusillo]

Summary

• add configurable repository setting expectations under githubSettings.expected
• report repository settings in github-repo-snapshot.sh --json without mutating GitHub
• surface deleteBranchOnMerge drift as a warning instead of assuming cleanup will happen
• record deleteBranchOnMerge: true as the expected setting for this repo

Refs #158

Validation

• github/scripts/validate-gh-issue.sh
• scripts/validate-skills.sh
• github/scripts/github-repo-snapshot.sh --json | jq '.github.repositorySettings'

[cbusillo]

Launchplane is building a merge-train candidate.

• Repository: cbusillo/codex-skills
• Base branch: main
• Pull request: Surface repo settings drift in snapshots #163
• Controller action: plan_landing
• Controller record: merge-train-batch-landing-plan-20260528T200702Z-3e6fde28dcb5bb7a

Launchplane is advancing plan_landing with merge-train-batch-landing-plan-20260528T200702Z-3e6fde28dcb5bb7a.

Launchplane manages this comment and will update it as the train moves.