fix(desktop): show remote channel agents in mentions

[sandeepgoenka]

What changed

• stop applying the local managed-agent custody filter to message mention candidates
• continue using the existing channel membership and relay invocation eligibility rules
• clarify regression coverage for a remote channel member without a local managed card

Why

Desktop mention autocomplete dropped agents hosted on another owned device before the newer eligibility rules ran. A live remote agent could therefore be a member of the current channel and still be absent from @ autocomplete simply because its private key was not stored on the viewing computer.

The local-only filter remains in the members-management surface, where local key custody is relevant. Message mentions now rely on the existing rules that show invocable agents and eligible channel members while hiding inaccessible non-members and explicitly non-invocable directory agents.

User impact

Users can mention a channel agent running on another device from Buzz Desktop without copying that agent's private key to the local device.

Validation

• just ci
• desktop unit suite: 3,906 passed
• desktop type-check and static checks
• Rust workspace and Tauri clippy/tests
• desktop and web production builds
• mobile analysis and 1,022 mobile tests