fix: make project_doc skill-render tests deterministic

[bolinfest]

Why

project_doc::tests::skills_are_appended_to_project_doc and project_doc::tests::skills_render_without_project_doc were assuming a single synthetic skill in test setup, but they called load_skills(&cfg), which loads from repo/user/system roots.

That made the assertions environment-dependent. After #11531 added .codex/skills/test-tui/SKILL.md, the repo-scoped test-tui skill began appearing in these test outputs and exposed the flake.

What Changed

• Added a test-only helper in codex-rs/core/src/project_doc.rs that loads skills from an explicit root via load_skills_from_roots.
• Scoped that root to codex_home/skills with SkillScope::User.
• Updated both affected tests to use this helper instead of load_skills(&cfg):
  • skills_are_appended_to_project_doc
  • skills_render_without_project_doc

This keeps the tests focused on the fixture skills they create, independent of ambient repo/home skills.

Verification

• cargo test -p codex-core project_doc::tests::skills_render_without_project_doc -- --exact
• cargo test -p codex-core project_doc::tests::skills_are_appended_to_project_doc -- --exact