Load project skills through worktree buffers (#58094)

Project-local skills now load through project buffers so remote
workspaces can resolve them consistently with other project files.

Validation:
- cargo check -p agent_skills -p agent
- cargo test -p agent_skills
- cargo test -p agent skills

Closes AI-329

Closes https://github.com/zed-industries/zed/issues/57877

Release Notes:

- Improved project skills support in remote workspaces.

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Co-authored-by: Cole Miller <cole@zed.dev>