[feat] MCP gateway: one managed entry point for MCP tools

[mmabrouk]

What

Agents can connect to MCP servers today, configured per agent. We want a managed MCP gateway: a single controlled entry point through which agents reach MCP tools, with connections and access managed centrally rather than re-configured on every agent.

Scope

• Register MCP servers centrally.
• Route an agent's MCP tool calls through the gateway.
• Central authentication and access policy for those servers.

Related earlier design work: #5067 (MCP delivery architecture).

Tracking issue for the public product roadmap.

[linear-code]

AGE-3998