[FEATURE]: Support subfolders for skills to better organize them

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

Problem: Skills are stored as a flat directory under ~/.config/opencode/skills/. As the number of custom skills grows, this folder becomes unwieldy. I have over 10 skills and the flat listing is getting messy fast.

opencode currently does not support subfolders under the skills directory — it only scans immediate children for SKILL.md files. Nesting skills into category subfolders (e.g., skills/testing/, skills/deployment/, skills/analysis/) breaks discovery.

Requested behavior: Allow skills to be organized into subfolders. opencode should recursively scan the skills directory (e.g., ~/.config/opencode/skills/*/) for SKILL.md files, or support an explicit nesting scheme so users can group related skills.

Use case example:

~/.config/opencode/skills/
  testing/
    unit-testing/SKILL.md
    e2e-testing/SKILL.md
  deployment/
    docker-deploy/SKILL.md
  analysis/
    code-review/SKILL.md
    debugging/SKILL.md

This mirrors the feature requested in github/copilot-cli#1632.

Locate the skills discovery entry point and inspect how it currently scans the skills directory for immediate-child SKILL.md files. Add coverage for the nested layout described in the issue, then verify that skills in category subfolders are discovered without breaking existing flat layouts.