Add experimental stdio upstream MCP support behind a feature gate

[davidruzicka]

Summary

Add experimental stdio upstream MCP support behind an explicit feature flag once the remote HTTP path and security boundary are in place.

Why

Stdio support is useful for local and self-hosted integrations, but it is more sensitive than remote HTTP because it introduces process lifecycle, environment injection, and execution isolation concerns.

Deliverables

• Add a gated stdio provider implementation disabled by default.
• Require explicit enablement through a feature flag or equivalent admin-only setting.
• Support command + args[] style process configuration without shell string execution.
• Define environment-variable injection by reference rather than inline secret values.
• Add lifecycle handling for startup timeout, shutdown, restart/failure behavior, and bounded resources.

Acceptance criteria

• Stdio upstreams cannot be enabled accidentally.
• Config validation rejects unsafe shell-style execution forms.
• Secrets remain env-referenced and redacted in logs/errors.
• Docs clearly mark stdio support as experimental and higher-risk.

Suggested files

• provider modules under src/
• src/types/profile.ts
• README.md
• docs/PROFILE-GUIDE.md
• stdio-focused tests

Agent-authored issue.

[github-actions]

🤖 Agent note (issuer)

Autonomy gate decision: unsafe
Reasons:

• issue includes explicit acceptance or validation structure
• issue matches high-risk keywords that should stay in a human lane