feat: add default .gitignore entries during init (#7)

[nmccready-tars]

Closes #7

Summary

During sync-agents init, automatically add sensible default .gitignore entries for agent tool directories.

Changes

• Added add_default_gitignore_entries() function that runs during init
• Adds .DS_Store if not already present
• Creates sync-agents section with ignore patterns for tool artifacts:
  • .cursor/* (except !.cursor/rules)
  • .codex/* (except !.codex/instructions.md)
  • .github/copilot/* (except !.github/copilot/instructions.md)
• Idempotent: Running init multiple times doesn't duplicate entries
• Smart updates: Detects and restores missing entries if section exists

Behavior

1. Creates .gitignore if it doesn't exist
2. Adds .DS_Store if not present (case-insensitive check)
3. Adds or updates sync-agents section with default patterns
4. Uses * + ! exception pattern to ignore tool artifacts while keeping symlinks tracked
5. Running init again updates section with any missing entries

Testing

Tested scenarios:

• ✅ Fresh repo (no .gitignore)
• ✅ Existing .gitignore with .DS_Store
• ✅ Running init multiple times (no duplicates)
• ✅ Manually removing entries and re-running init (entries restored)

Symlinks remain tracked in git while tool-generated artifacts are ignored.