mecatui: classify skills inventory bounded viewport migration #1898

Scope

Classify and, if appropriate, migrate the /skills inventory in cmd/mecatui/ui/skills.go to bounded.Viewport.

Current behavior

A single wrapped physical document combines external skills and learned-skill rows. It has an explicit scroll offset, filter input, wheel handling, and a cursor/Enter action that applies only to learned skills.

Required outcome

• First establish whether the mixed interaction remains a valid bounded.Viewport fit; retain it explicitly if it does not.
• If migrating, preserve filter routing, width cache, wheel behavior, learned-skill cursor/Enter behavior, and the distinction between learned and external entries.
• Do not force this mixed surface into bounded.List merely to share a primitive.

Verification

Add scenario-first acceptance coverage for the approved classification and run focused skills/UI and bounded tests before project gates.