Set Obsidian client keep-alive timeout

[fa1k3]

Summary

• set explicit undici Agent keep-alive timeouts for the Obsidian REST client
• keep the client idle timeout below the Local REST API server keep-alive timeout
• add regression coverage for the Agent option builder

Why

Short, explicit client-side keep-alive avoids stale-socket reuse races against upstream servers with short keep-alive windows.

Tests

• bun test tests/services/obsidian-service.test.ts
• bun run test
• bun run build
• bun run lint:mcp