feat(tui): skills configuration panel — add, edit, remove skills

[EngineerProjects]

Summary

A TUI panel (accessible via ctrl+p → Skills) to manage the project's
.nexus/skills/ files without leaving the terminal. Users can list
active skills, add new ones from a template, edit existing ones, and
toggle them on/off per session.

UX

┌─ Skills ──────────────────────────── ✓ 3 active ───┐
│  / filter█                                          │
│  ──────────────────────────────────────────────     │
│  ▶ go-conventions        ✓ active   project         │
│    git-workflow           ✓ active   project         │
│    security-rules         ✗ disabled project         │
│  ──────────────────────────────────────────────     │
│  + New skill                                        │
│  ──────────────────────────────────────────────     │
│  enter: edit  space: toggle  n: new  del: delete    │
└─────────────────────────────────────────────────────┘

Acceptance criteria

• List all skills from .nexus/skills/ with active/disabled status
• Toggle skill on/off for the current session (space bar)
• Open skill in $EDITOR from within the panel
• Create new skill from a template (name prompt → creates file)
• Delete skill with confirmation prompt
• Filter by name