Add MCP streamable_http transport, integrate gnexus-creds, and document headless nodes ...

- navi/mcp/client.py: add streamable_http transport via httpx + mcp.client.streamable_http
- navi/mcp/config.py: add "streamable_http" to transport literal and is_streamable_http property
- mcp_servers.d/gnexus-creds.json: new MCP server config with overlay instructions for secret workflow
- docs/future_headless_nodes.md: architecture exploration for headless Navi node swarm

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>