refactor(opencode): fetch remote config with http client

[kitlangton]

Summary

• fetch well-known and remote config JSON through injectable Effect HttpClient
• keep HttpClient optional so local-only config loading does not require HTTP
• update remote config tests to use injected HttpClient instead of global fetch mocks

Verification

• bun test --timeout 5000 test/config/config.test.ts
• bun test --timeout 5000 test/plugin/trigger.test.ts
• bun test --timeout 5000 test/plugin/workspace-adapter.test.ts
• bun test --timeout 5000 test/agent/plugin-agent-regression.test.ts
• bun typecheck
• bunx oxlint packages/opencode/src/config/config.ts packages/opencode/test/plugin/trigger.test.ts packages/opencode/test/plugin/workspace-adapter.test.ts packages/opencode/test/agent/plugin-agent-regression.test.ts