Desktop: allow adding an existing local agent without starting it

[ismcaldwell]

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.
• ManagedAgentRecord: add disable_local_spawn flag to prevent Desktop from reactively spawning an agent whose runtime is externally managed #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.

[ismcaldwell]

Implementation is ready for maintainer review in #7205. It adds a visible Add without starting action for stopped local managed agents, verifies stopped state after membership, preserves the existing add-and-start path, and includes pointer/keyboard regression coverage plus before/after screenshots. The exact source head is 588efb71409cf96a0492921686c63796b9199fa3.