Add merge rules engine for validating PRs before landing

[seemethere]

Introduces a new merge_rules module that provides:

• MergeRule dataclass for rule configuration (patterns, approvers, checks)
• MergeRulesLoader to load rules from .github/merge_rules.yaml
• MergeValidator to validate PRs against rules including:
  • File pattern matching (fnmatch/glob)
  • Approver validation with team expansion (org/team-slug)
  • CI check status validation
• ValidationResult and MergeValidationError for error handling

Also adds new GitHub API methods to GitHubEndpoint:

• get_pr_reviews, get_pr_files, get_check_runs
• get_team_members, get_file_contents, post_issue_comment

Adds pyyaml dependency for YAML parsing.

[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