A minimal built-in Streamable HTTP MCP server, for tests and for operators
kicking the tyres on reactive setups. Hidden mode:
agentd --internal-mock-mcp-http <addr-file> <uri> [--no-emit].
Build an MCP client from a declared crate::config::McpServerSpec: resolve
its secret-free {{secret:…}} auth header templates (via auth) and connect
to the spec’s remote endpoint, stamping agentd’s client identity. The
config/auth-coupled counterpart of the crate’s transport-only
client::McpClient::connect. Call initialize on the result before use.