Add recursive directory scanning to discover skills in nested directories
like superpowers (e.g., skills/superpowers/brainstorming/SKILL.md).
Changes:
- Add namePrefix, depth, and maxDepth parameters to loadSkillsFromDir
- Recurse into subdirectories when no SKILL.md found at current level
- Construct hierarchical skill names (e.g., 'superpowers/brainstorming')
- Limit recursion depth to 2 levels to prevent infinite loops
This enables compatibility with the superpowers plugin which installs
skills as: ~/.config/opencode/skills/superpowers/ -> superpowers/skills/
Fixes skill discovery for nested directory structures.
64b29ea097