[FEATURE]: Add preview button to skills list #21686

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently, when running /skills in the TUI, users see a list of skills with names and descriptions but cannot preview the full SKILL.md content before invoking a skill.

Request: Add a "Preview" button to each skill in the /skills dialog that opens a modal displaying the full SKILL.md content with:

• Markdown rendering (code blocks, tables, syntax highlighting)
• Skill metadata (name, description, source path)
• Optional actions: Copy to Clipboard, Invoke Skill

Benefits:

• Better discoverability. Users understand what each skill actually does
• Reduces trial-and-error when selecting skills
• Users can read full instructions before invoking
• Consistent with /models, /sessions patterns

Implementation approach:

• Add preview button to existing DialogSkill component
• Create preview modal using MarkdownRenderable from @opentui/core (same component used for AI responses in TUI)
• Use existing Skill.Info.content field (already returned by SDK's app.skills() endpoint)
• No new dependencies required

Additional note:
I'm happy to implement this feature myself. Happy to discuss the approach before starting implementation.