Support Sapling SCM (.sl) for project root detection

Problem

Claude Code uses .git to detect the project root, which drives feature discovery (skills, CLAUDE.md, project settings). Repositories using Sapling SCM in native mode (.sl directory instead of .git) are not detected as projects.

This means:

• Custom skills in .claude/skills/ are not discovered (don't appear in /skills)
• Project-level CLAUDE.md may not be found reliably
• Is a git repository: false in the environment info even though it's a valid VCS checkout
• Any other feature gated on project root detection is broken

Expected behavior

Claude Code should detect .sl as a valid VCS marker alongside .git when determining the project root. Sapling is a Git-compatible SCM developed at Meta and used in production — repos can operate in either .git mode (coexists with git) or native .sl mode.

Workaround

Users can run Sapling in "dotgit" mode where it maintains a .git directory, but this shouldn't be required — native .sl checkouts are a supported Sapling workflow.

References

• Sapling SCM: https://sapling-scm.com/
• Sapling GitHub: https://github.com/facebook/sapling