feat(converter): add Claude Code to OpenCode content transformation (#36)

Add comprehensive CC→OC migration capabilities to the converter module:

- Transform skill/command frontmatter by removing CC-only fields
  (model, allowed-tools, disable-model-invocation, user-invocable, context, agent)
- Transform command frontmatter: remove argument-hint, normalize model field
- Transform body content: convert tool names (Task→delegate_task, TodoWrite→todowrite, etc.),
  update path references (.claude/→.opencode/), replace prefix patterns
- Add skipBodyTransform option to disable body transformations
- Include skipBodyTransform in cache key for proper invalidation
- Widen frontmatter typing (Record<string, unknown>) to preserve arrays/objects
- Use js-yaml dump for robust frontmatter serialization (prevents corruption of
  non-scalar YAML values)
- Add comprehensive test coverage for all transformation scenarios

Also adds:
- docs/CONVERSION-GUIDE.md: Complete CC→OC migration reference for humans and AI agents
- Test cleanup: Replace non-null assertions with explicit guards to satisfy Biome lint rules

Fixes an edge case where formatFrontmatter() would corrupt complex YAML structures
(arrays/objects) by switching from string interpolation to proper YAML serialization.

Author: marcusrbrown