Desktop: allow adding an existing local agent without starting it

**Motivation**

A channel owner may want to add an existing local managed agent to a channel while intentionally keeping its runtime stopped. The current Desktop member flow starts a stopped local agent while attaching it, coupling channel membership with runtime execution.

**Proposed solution**

Add a clear owner-visible choice in the existing local-agent member flow to add the agent without starting it. The path should use the existing non-starting helper option, publish membership only, preserve the stopped state, and show a visible error if attachment fails. Normal add-and-start behavior can remain available as a separate choice.

The new control should support pointer and keyboard use and have an accessible label.

**Alternatives considered**

- Add normally and stop afterward: the process has already started, so this does not preserve the user intent.
- Use an internal command or developer tool: this would not provide a supported owner-facing product workflow.
- #6468: related but different; that proposal prevents reactive local spawning for externally managed runtimes. This request concerns an ordinary local managed agent that should remain stopped until the owner starts it.

**Additional context**

#4835 is also related but concerns finding and re-adding a removed personal agent. No matching open pull request or exact issue was found for preserving stopped state during attachment.