Add multi-agent routing and deployed-agent version selection

[shibayan]

Priority

P2

Goal

Allow one host process to expose multiple configured agents and select the correct local or deployed agent using validated request metadata.

Scope

• Add an agent registry or keyed agent factory.
• Select agents by documented agent name, entity identifier, or version metadata.
• Define fallback and unknown-agent behavior.
• Preserve per-agent configuration, identity isolation, and telemetry.

Acceptance criteria

• One server can route requests to at least two agents.
• Unknown or unauthorized agent selections fail explicitly.
• Concurrent calls cannot leak state between agents.
• Deployed-agent version selection is represented without changing existing simple configuration.
• Routing has integration tests and documentation.

Dependency

Blocked by the common hosted execution context.