Implement the Agent Skills standard (agentskills.io) for extending agent capabilities with procedural knowledge packages.
- Add Skill module with discovery, parsing, and validation
- Load skills from .opencode/skill/ (via Config.directories)
- Load skills from .claude/skills/ (via Filesystem.up)
- Inject available skills into system prompt as XML block
- Add 11 tests covering discovery, validation, and error cases
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