fix(ui): remove misleading sample Skill action

[jackwener]

Summary

• remove the Skills-page Add button that silently created a sample Skill
• remove the same sample-generation action from the installed-empty state
• delete the now-dead renderer action, props, toast copy, localization, and stale styling commentary

UX rationale

The primary action did not map to a real user goal: Add implied selecting or installing a real Skill, while the result was generated example content. Real Skills remain available through built-in installation, source import, filesystem discovery, and refresh.

Verification

• reviewed against the latest origin/main
• Biome check passed for all 9 changed files
• no full test suite run for this UI subtraction