feat(config): support .claude/rules/*.md instruction files

[peteroden]

What

Claude Code supports modular instruction files in .claude/rules/*.md with path-scoping via YAML frontmatter (paths: ["src/**/*.ts"]). Load these as additional project instructions in repo_config.py.

Acceptance Criteria

• discover_repo_config() loads .claude/rules/*.md files
• Frontmatter paths field is parsed and available for path-scoped application
• Instructions are appended to the system prompt alongside existing instruction sources