Start session should preselect the workspace default agent #667

What problem would this solve?

A workspace can declare default_agent, but the generic Start session dialog opens with general selected. For a workspace whose default is another agent, the preselection disagrees with its configuration and can block creation when general is unavailable. This also affects a Start session action launched from a worktree.

What would you like to happen?

Preselect the active workspace's default_agent for an unspecified Start session action. A worktree launch should use its owning workspace's default. Agent-specific actions should keep their explicitly selected agent. When no workspace default is configured, retain the current fallback and require a valid selection before creating a session.

Area

Sessions, Workspaces, Web UI

Alternatives considered

Selecting the agent manually on every launch works, but does not honor the workspace setting.