Network transfer of agents between owner devices. Agent migration

**Motivation**
I use 4 computers for development work. This would make it a lot easier to move between computers.

**Proposed solution**

Add Settings → Agents → Import from another computer.
Show the owner’s authenticated/online Buzz devices.
Let the destination select a source device and require approval on the source.
Transfer the encrypted agent configuration over the authenticated relay connection, without an intermediate file.
Include name, system prompt, runtime/model settings, tools, permissions, and identity metadata.
Keep credentials/secrets device-local, or require re-authorization on the destination.
Offer preserve identity versus clone as a new identity.
Show a review/confirmation screen and handle name/identity conflicts clearly.

**Alternatives considered**
Manual migration.

**Additional context**
Anything else that helps — links, screenshots, prior art.

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".