Preview the highlighted skill's description in the picker

[xsh310]

What

A small UX follow-up on the ug skill add picker (PR #579): the picker now previews the highlighted skill's description in a footer, so a row is more than a bare FQN. Requested in review on #579.

Changes

• SkillRef gains a description field, populated by _skill_ref from the ListSkills response, so it rides in on the same streaming walk with no per skill GetSkill.
• _skill_download_choice passes it through as the Choice.description.
• scrolling_checkbox gains show_description (default False, so the MCP picker is unchanged). When on, it renders the highlighted row's description in its own footer window, updating as you move the cursor.

Why a footer rather than questionary's inline show_description

questionary's built-in show_description appends the description as an extra line inside the choices window. That window is sized to the row count in this picker, so the inline line would be clipped. A dedicated footer window (mirroring the existing search string and loading footers) sidesteps that, and showing only the highlighted row's description means a long one (UC caps skill descriptions at ~1024 chars) is confined to a bounded, 2 line wrapping footer via _description_preview, instead of bloating every row.

 › main.default.triage
   main.default.pii
   ml.prod.scoring
 Routes incoming support tickets to the right on call queue by severity.   ← footer: highlighted row

Testing

uv run ruff check ., ruff format --check src tests, ty check src, and uv run pytest all pass (the 2 pre-existing e2e failures in test_e2e_user_agent and test_claude_smart_routing_v2 are unrelated and also fail on the base branch). New unit tests cover _description_preview boundaries (short unchanged, at the limit unchanged, long clipped with an ellipsis), _skill_ref carrying the description, the Choice carrying it, and the picker being invoked with show_description=True.

Stacked on #579.

Screen.Recording.2026-09-12.at.12.01.57.AM.mov

This pull request and its description were written by Isaac.

[sunishsheth2009]

P2 (UX): the description footer window is fixed at Dimension.exact(2). A UC description containing embedded newlines will have its later lines clipped even when it's well under the 240-char budget. UC descriptions are usually single-line, so minor — but consider collapsing whitespace/newlines into spaces before preview so the 2-line budget is spent on content rather than a hard-wrapped first line.