feat(core): discover ecosystem skill directories

[jlongster]

Summary

• discover global and project-level .agents and .claude directories
• load skills from their skills subdirectories
• keep non-skill config plugins scoped to OpenCode directories

Testing

• bun test test/config/config.test.ts (18 passed)
• git diff --check
• packages/server: bun typecheck

Known validation issue

• test/config/skill.test.ts aborts during module initialization with an existing SkillV2.ID circular initialization error before tests run
• full pre-push typecheck was not completed because TypeScript processes were repeatedly OOM-killed in the local environment