refactor: remove ConnectionGuardian and all MCP coupling

The plugin now operates as a pure native plugin like Morph — no MCP
server dependency. Tools call Nia API directly via NiaClient HTTP.

- Remove ConnectionGuardian class and tests
- Strip mcpServerName/mcpMaxRetries/mcpReconnectBaseDelay from config
- Clean MCP fields from all 14 test configs
- Bump to 0.2.1