Make Home the main screen + disable Dashboard for agent-enabled workspaces

For workspaces that have the new agent enabled (workspaces.settings.agent_v2_enabled), make the Home screen (agent/composer home) the actual main landing screen, and disable the old Dashboard for them.

Scope:

* Agent-enabled workspaces: Home becomes the default route/landing; Dashboard route + sidebar entry hidden/disabled (redirect Dashboard → Home).
* Non-agent workspaces: keep the current Dashboard-as-home behaviour unchanged.
* Gate on agent_v2_enabled (per-workspace, no agency inheritance — only flagged workspaces are affected).

Not started — tracking only.