feat(desktop): support owner-reviewed local team snapshot imports

[marcusquinn]

Motivation

Buzz Desktop owns local agent and team state, but external tooling currently has no supported way to submit a deterministic team snapshot for owner review. The alternatives are manual repetition, undocumented state-file edits, or fragile accessibility automation. This is a focused first implementation slice of the broader secured Desktop control proposal in #4869.

Proposed solution

Add a versioned, same-user local Desktop control endpoint and matching buzz desktop commands with a deliberately narrow initial surface:

• buzz desktop status --json reports whether the running Desktop supports the protocol;
• buzz desktop import-team-snapshot submits a bounded snapshot with an idempotency key;
• Desktop stores the request durably, brings the existing snapshot-review UI forward, and leaves the owner to approve or reject the exact import;
• the endpoint never returns private keys or provider credentials and never bypasses existing validation or review.

On Unix platforms, the endpoint should use a user-only 0600 Unix-domain socket, reject non-owner peers, cap request size and read time, and return structured protocol errors. Unsupported platforms should fail explicitly rather than exposing a weaker transport.

Alternatives considered

• Accessibility automation is possible but brittle and cannot guarantee exact configuration or idempotency.
• Editing Desktop state files bypasses validation and keyring boundaries.
• Relay-side drafts do not provide durable receipt into the local Desktop review flow.
• A general remote HTTP administration API would create a much larger attack surface.

Additional context

This issue intentionally limits #4869 to owner-reviewed team snapshot imports and protocol status. Runtime registration, lifecycle management, settings, and Windows transport can follow separately after the security boundary and review flow are validated.

Proposed verification:

• protocol framing, peer authorization, permissions, timeouts, and payload limits;
• durable pending-import and idempotency behavior;
• CLI status/import success and structured failure cases;
• full repository CI (./bin/just ci).

Closest existing issue: #4869. No duplicate implementation PR was found.

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