IRCX v1.10.2: cleaner .agent errors + temporary model swaps

Patch fix for the .agent command bridge.

• No more leaked slash forms. An unknown .agent <x> used to reply "Unknown command /x"; it now answers "unknown command: .agent x. Try '.agent help'", validated against the gateway registry before dispatch.
• Model swaps are temporary by default. .agent model <name> now applies for the current session only (no config write). Persist explicitly:
  • .agent model <name> --persist saves to this profile's config.yaml
  • .agent model <name> --global saves to the shared config (all profiles)
  • .agent model <name> --session is explicit session-only (the default)
  • a bare .agent model still shows the current model

Note: Hermes' /model has no --persist flag of its own (it persists by default, with --session/--global modifiers); --persist here is provided by the bridge and maps to a profile-scoped save.