Desktop republishes agent kind-0 profiles from managed-agent record, clobbering agent-set avatar/about

🤖 *Filed by Sam Adriance's AI agent on his behalf.*

## Bug

Buzz Desktop republishes each managed agent's kind-0 profile from its managed-agent record (`managed-agents.json`), overwriting profile fields the agent set itself via `buzz users set-profile`.

## Repro (observed)

1. A managed agent runs `buzz users set-profile --avatar ` — the relay profile updates and the new avatar shows in chat. ✔️
2. Later (agent restart / record save), Desktop republishes the profile sourced from the record's `avatar_url`.
3. The CLI-set avatar is clobbered back to the record's value. Confirmed by diffing the kind-0 `picture` against the record over time.

The `about` field is exposed to the same mechanism: the managed-agent record has **no** about/description field, so a republish sourced from the record either drops or wipes an agent-set bio (bios set via CLI currently work, but nothing in the record preserves them across a republish).

## Expected

One of:
- Desktop merges rather than replaces: republish only the fields it owns, preserving `about` (and any field absent from the record), or
- The record becomes the full source of truth *and* the create/edit agent form exposes avatar + description so owners can actually set them durably.

## Environment

- Buzz Desktop on macOS (Darwin 25.5.0), Tauri build

Start by tracing Desktop's managed-agent profile republish flow and the managed-agents.json record, then compare it with the `buzz users set-profile` path for kind-0 `picture` and `about` fields. Check the create/edit agent form and related persistence behavior. Done means a later restart or record save no longer removes agent-set fields, or the form and record durably own those fields.

Written by the indexing model from the issue text.