test(opencode): migrate tool registry fixtures

[kitlangton]

Summary

• add ToolRegistry.use service accessors
• migrate registry test fixture writes to AppFileSystem.use.writeWithDirs
• keep the zod fixture copy as a Promise boundary for fs.cp

Verification

• bunx prettier --write packages/opencode/test/tool/registry.test.ts packages/opencode/src/tool/registry.ts
• bun typecheck (packages/opencode)
• bun typecheck (packages/core)
• bun test --timeout 5000 test/tool/registry.test.ts (packages/opencode)
• bunx oxlint packages/opencode/src/tool/registry.ts packages/opencode/test/tool/registry.test.ts (warnings only: pre-existing unsafe assertion/unbound-method/no-unneeded-ternary warnings)