feat: add custom skills path configuration

[anntnzrb]

Summary

• Add skills.path config option to opencode.json
• Support additional skill directories with ~ expansion
• Scan custom paths in addition to default locations
• Add documentation and tests

Usage

{
  "skills": {
    "path": ["~/my-skills", "~/.config/agents/skills"]
  }
}

Partially addresses #6266 - allows adding custom directories for skill discovery.

Note: This is additive only. Disabling/replacing default paths would be a separate feature if needed.