Add real stdio MCP process wrapper

Add a minimal runtime stdio MCP launcher that spawns configured server processes with piped stdin/stdout, applies transport env, and exposes async write/read/terminate/wait helpers for future JSON-RPC integration.

The wrapper stays intentionally small: it does not yet implement protocol framing or connection lifecycle management, but it is real process orchestration rather than placeholder scaffolding. Tests use a temporary executable script to prove env propagation and bidirectional stdio round-tripping.

Constraint: Keep the slice minimal and testable while using the real tokio process surface
Constraint: Runtime verification must pass cleanly under fmt, clippy, and tests
Rejected: Add full JSON-RPC framing and session orchestration in the same commit | too much scope for a clean launcher slice
Rejected: Fake the process wrapper behind mocks only | would not validate spawning, env injection, or stdio wiring
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Layer future MCP protocol framing on top of McpStdioProcess rather than bypassing it with ad hoc process management
Tested: cargo fmt --all; cargo clippy -p runtime --all-targets -- -D warnings; cargo test -p runtime
Not-tested: live third-party MCP servers; long-running process supervision; stderr capture policy

rust/crates/runtime/src/lib.rs

@@ -7,6 +7,7 @@ mod file_ops;
 mod json;
 mod mcp;
 mod mcp_client;
+mod mcp_stdio;
 mod oauth;
 mod permissions;
 mod prompt;