Add --validate-rules, --dry-run, --comment-on-failure to land command

[seemethere]

Extends ghstack land with merge rules validation:

CLI options:

• --validate-rules: Validate PRs against .github/merge_rules.yaml
• --dry-run: Validate without actually merging
• --comment-on-failure: Post validation errors as PR comments

The validation happens before any git operations:

1. Load merge rules from repository
2. For each PR in the stack, validate approvers and CI checks
3. If any PR fails validation, abort with detailed error
4. Optionally post error details as a PR comment

Example usage:
ghstack land --validate-rules --comment-on-failure <PR_URL>

[seemethere]

Stack from ghstack (oldest at bottom):

• Add example workflows and documentation for ghstack merge #320
• Add GitHub Actions reusable workflow for @ghstack merge #319
• -> Add --validate-rules, --dry-run, --comment-on-failure to land command #318
• Add merge rules engine for validating PRs before landing #317