feat: add validate command and dry-run mode for sync/clean

[sonwr]

Summary

This PR adds two usability features to heymark CLI:

1. validate command for skill frontmatter/name validation
2. --dry-run mode for sync and clean commands

What changed

• Added new command: heymark validate
  • checks required description
  • checks alwaysApply type is boolean when provided
  • checks globs type is string when provided
  • checks duplicate skill names
  • exits with non-zero code on validation failures
• Added dry-run flags for sync/clean:
  • --dry-run and -n
  • sync dry-run prints planned generation without writing files
  • clean dry-run prints planned cleanup without deleting files
• Added dry-run support in shared tool generators/cleaners (HEYMARK_DRY_RUN=1).
• Updated help output and README command examples.

Validation

Executed locally:

• node src/index.js help
• node src/index.js link https://github.com/MosslandOpenDevs/heymark.git --folder skill-samples
• node src/index.js validate
• node src/index.js sync . --dry-run
• node src/index.js clean . --dry-run

All commands ran successfully with expected output.

[yezzero]

sync 명령어가 따로 있으므로
validate나 clean을 할 때는 sync를 원하지 않을 수도 있을 거 같아 수정을 해봤습니당

예상 시나리오:
1. 원격에 있는 스킬들을 sync해두고
2. 로컬에서 상황에 맞춰 수정해뒀는데 (ex: 이 프젝에서만은 이 스킬이 alwaysApply: false여야 할 때)
3. validate/clean 호출하면 모르는 사이에 git pull이 되어 어리둥절

더 불편할 것 같다면 없애겠습니당