The runtime crate already had typed MCP config parsing, bootstrap metadata,
and stdio JSON-RPC transport primitives, but it lacked the stateful layer
that owns configured subprocesses and routes discovered tools back to the
right server. This change adds a thin lazy McpServerManager in mcp_stdio,
keeps unsupported transports explicit, and locks the behavior with
subprocess-backed discovery, routing, reuse, shutdown, and error tests.
Constraint: Keep the change narrow to the runtime crate and stdio transport only
Constraint: Reuse existing MCP config/bootstrap/process helpers instead of adding new dependencies
Rejected: Eagerly spawn all configured servers at construction | unnecessary startup cost and failure coupling
Rejected: Spawn a fresh process per request | defeats lifecycle management and tool routing cache
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep higher-level runtime/session integration separate until a caller needs this manager surface
Tested: cargo fmt --all; cargo clippy -p runtime --all-targets -- -D warnings; cargo test -p runtime
Not-tested: Integration into conversation/runtime flows outside direct manager APIs
87b232fa0d