[SaladDay]

Public stdio MCP servers now run natively wherever the Harness can start one in the declared cwd.

• Codex writes command, args, cwd and env_vars to its native mcp_servers entry, and its post-trust check verifies all four.
• Claude Code starts stdio servers in its workspace directory, so cwd must equal it, and it needs allowed_tools: null, because native hides unselected tools only for HTTP servers.
• MiniMax Code rejects every stdio server with a typed error. Its ACP session servers carry no cwd, and native 0.6.10 falls back to homedir(), which the adapter points at private state.
• Admission: a stdio server needs the origin environment and an execution Environment. It takes no credential_id.
• Single source: a nonempty inline env and an inline authorization are rejected. Values come only from the Environment (env_vars), and credentials only from Vaults (credential_id). index.md records both as differences from OpenAI.
• Wire: MCPHTTPServer gains an optional Stdio declaration. MCPTransport replaces the generated HTTP-only transport in contracts/agents-api/v1.
• Protocol version: this PR and Let Subagents call function and MCP tools #780 both set 0.20.0, and whichever merges second bumps to 0.21.0.

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