Add librarian managed skill

[iancleary]

Summary

• add the librarian skill as a Forge-managed skill with its checkout.sh helper
• register and embed the skill in the Forge release skill payload
• route general Forge skill selection and AGENTS guidance to use librarian for remote git repository references, except while developing Forge itself

Verification

• cargo run -p forge -- --json skills validate --all --source repo --repo-path /home/iancleary/.openclaw/workspace/forge --check-upstream
• cargo test -p forge embedded_release_skills
• cargo test -p forge embedded_release_skill_frontmatter_names_match_release_names
• cargo check
• cargo run -p forge -- --json skills diff librarian --source repo --repo-path /home/iancleary/.openclaw/workspace/forge --target user
• cargo run -p forge -- --json skills diff forge-tools --source repo --repo-path /home/iancleary/.openclaw/workspace/forge --target user
• /home/iancleary/.agents/skills/librarian/checkout.sh https://github.com/mitsuhiko/agent-stuff/blob/main/skills/librarian/SKILL.md --path-only