[FEATURE]: explicit skill sources and source-qualified duplicates

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I use separate skill trees for multiple agent harnesses and want to compare them independently. OpenCode automatically scans compatibility directories such as ~/.claude/skills and ~/.agents/skills. When both trees contain a skill with the same directory name, OpenCode reports duplicate-name warnings and can fail while building the system prompt with:

TypeError: undefined is not an object (evaluating 'a.name')
at SystemPrompt.environment

Reproduction on macOS with OpenCode from the dev branch:

1. Create ~/.claude/skills/review/SKILL.md.
2. Create ~/.agents/skills/review/SKILL.md.
3. Start OpenCode in a project that can discover both roots.
4. Observe duplicate-name warnings and, depending on discovered ordering, a system-prompt construction failure.

Please support explicit skill sources and source-qualified IDs:

{
  "skills": {
    "sources": [
      { "id": "opencode", "path": "~/.config/opencode/skills", "enabled": true },
      { "id": "agents", "path": "~/.agents/skills", "enabled": true },
      { "id": "claude", "path": "~/.claude/skills", "enabled": true }
    ],
    "collision": "source-qualified"
  }
}

Expected behavior:

• Explicit sources control which roots are scanned.
• Same-named skills from different sources coexist as claude/review and agents/review.
• Discovery and selection are deterministic.
• Duplicate or malformed skill files produce diagnostics without crashing.
• Existing behavior remains unchanged when skills.sources is not configured.
• Source IDs support isolated harness and version A/B comparisons.

This complements related reports #31977, #32202, #7575, #25119, and #41288.