feat: add skills settings and /skill picker

[imodeveloper]

Summary

• add a Settings → Skills view with Use/Open/Edit actions
• add /skill slash command picker that inserts skill names in the prompt
• add styles and tests for skills UI and autocomplete

Motivation

Surface installed skills directly in CodexMonitor and make them easy to insert with /skill, matching the Codex app workflow.

Testing

• npm run test
• npm run typecheck

[imodeveloper]

Fixes skill edit failures by trying nested /SKILL.md when needed and sharing worktree skills via parent cwd.

Tests:

• npm run test -- SkillsView.test.tsx
• cargo check (src-tauri)

[imodeveloper]

Test run:

• npm run test
• npm run typecheck

[imodeveloper]

Edit now resolves relative skill paths (e.g. .codex/skills/...) via home dir expansion.

Test:

• npm run test -- SkillsView.test.tsx