[FEATURE]: display loaded skills in status page and sidebar

[connorads]

Problem

Loaded skills aren't visible in the TUI. Users can run opencode debug skill to see them, but having this info in the UI would be more discoverable.

Proposed solution

Display skills list in:

• Status page
• Sidebar (collapsible when multiple skills)

Fix: https://github.com/anomalyco/opencode/pull/6154