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.

[anntnzrb]

Fixed CI failure:

Test fix: The test was calling Config.update() inside Instance.provide, but Skill.state is cached via Instance.state - it reads config at initialization time. Restructured to write opencode.json in tmpdir.init callback (before instance starts).

Code fix: Custom directories now use OPENCODE_SKILL_GLOB ({skill,skills}/**/SKILL.md) instead of CLAUDE_SKILL_GLOB, giving users flexibility to use either skill/ or skills/ subdirectory naming.

Docs fix: Clarified that custom paths must contain a skill/ or skills/ subdirectory with skills inside.

The Bedrock test timeout is a pre-existing flaky test unrelated to this PR.

[airtonix]

@anntnzrb you might like my plugin instead https://github.com/zenobi-us/opencode-skillful