[FEATURE] Add --project-only option to sync command

[boyonglin]

Summary

Add a --project-only (or --local) flag to the sync command that only syncs project-level skills to AGENTS.md, excluding global skills.

Use Case

When working on a specific project, I only want the project's AGENTS.md to contain skills relevant to that project. Currently, npx openskills sync includes both global (~/.claude/skills/) and project (./.claude/skills/) skills, which can make AGENTS.md very large and cluttered with unrelated skills.

Proposed Solution

Add a new option to the sync command:

npx openskills sync --project-only
# or
npx openskills sync --local

This would only scan and sync skills from the project's skills directory, ignoring the global ~/.claude/skills/ directory.

Current Workaround

Temporarily rename the global skills folder
Run npx openskills sync
Restore the global skills folder
This is cumbersome and error-prone.

Additional Context

Current sync options: -y, --yes and -o, --output
The interactive mode allows manual deselection, but this is tedious when there are 100+ global skills
Thank you for this great tool! 🙏