test(skill): migrate skill tests to effect fixtures

[kitlangton]

Summary

• migrate skill tests to it.instance fixtures and AppFileSystem.use writes
• add Skill.use service accessor for cleaner Effect-native calls
• preserve existing skill test names and assertions

Verification

• bunx prettier --write packages/opencode/test/skill/skill.test.ts packages/opencode/src/skill/index.ts
• bunx oxlint packages/opencode/src/skill/index.ts packages/opencode/test/skill/skill.test.ts
• bun typecheck (packages/opencode)
• bun test --timeout 5000 test/skill/skill.test.ts (packages/opencode)
• test names unchanged (16)