Add path-scoped rules feature

[HartBrook]

Summary

• Add path-scoped rules system that syncs to ~/.claude/rules/
• Rules use YAML frontmatter with paths: to scope guidelines to specific file patterns
• Implement precedence model: project > personal > team > starter
• Include 5 starter rules: security, testing, api/rest, frontend/react, error-handling

Changes

Core rules package (internal/rules/)

• rule.go - Rule parsing with frontmatter support and ParseErrors type
• registry.go - Precedence-based rule registry with deterministic ordering
• claude.go - Convert rules to Claude Code format preserving frontmatter

Sync integration (internal/cli/sync.go)

• Recursive fetching of rules from team repos
• Sync to ~/.claude/rules/ with collision detection
• Add --rules-only flag for isolated rule syncing
• Fix os.RemoveAll error handling

Starter rules (internal/starter/rules/)

• Security guidelines (secrets, input validation, auth)
• Testing standards (coverage, quality, organization)
• REST API standards (HTTP methods, status codes, error responses)
• React guidelines (components, state, performance, accessibility)
• Error handling patterns

Documentation

• Add Path-Scoped Rules section to README
• Update repository structure, file locations, CLI flags
• Update example/README.md with rules structure

Test plan

• Unit tests for rule parsing, registry, and conversion
• Integration tests for sync, precedence, and subdirectories
• All existing tests pass
• Lint checks pass