fix(desktop): add existing remote agents to channels

[timigod]

Summary

• separate existing relay-agent identities from local persona deployment in Add agents
• let a client with no runtimes attach the canonical existing identity to a channel
• do not mint another keypair or require a local provider for that path
• add a focused Playwright regression

Why

The current Add agents dialog is persona-first and disables its action when the client has no runtime provider. On a chat-only second device, that prevents adding an agent which already exists and is running elsewhere; enabling a provider would instead risk creating a duplicate identity.

Buzz already has the identity-level relay primitive. This change exposes that existing-agent path directly while leaving local persona deployment intact.

Related: #3639 and #6232.

Verification

• desktop TypeScript typecheck
• Biome/check scripts for the affected client files
• all 5,556 desktop unit tests in the combined patch tree
• focused Playwright: a client without runtimes can add an existing remote agent