Summary

- Allow write_file and edit_file to create or modify files under
~/.agents/skills.
- Keep the global skills exception constrained to that directory and
preserve existing project-path behavior.
- Document global skill file editing support in the built-in
create-skill instructions.

Tests

- cargo fmt -p agent
- cargo test -p agent global_skill_file
- cargo test -p agent
test_create_directory_allows_global_skill_directory

Release Notes:

- Fixed agent file editing for global skills

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Co-authored-by: Richard Feldman <oss@rtfeldman.com>