Plugin-shipped agents now appear in the agent picker

Installing a package that ships its own AI agents registered those agents in your settings — but the chat's agent picker never read that registration, so the agents stayed invisible unless you happened to chat inside the package's own space.

The picker, the agent combobox and the chat engine now resolve your registered agent sources everywhere: a package's agents are offered right next to the built-in and your own agents, in any space, and the selection validates against the same list the chat actually executes with.