Adds Vitest + TypeScript test infrastructure for agent validation. Validates 642 agent files across 14 categories for YAML frontmatter, kebab-case naming, and directory population.
Fixes Copilot agent install path (copies to both ~/.github/agents and ~/.copilot/agents for backwards compatibility) and OpenCode directory handling (searches both flat and nested layouts). Closes#218, #228, #185, #245.