Buzz resets after picking a local agent to run in it.

**Describe the bug**
Selecting certain models in Share compute causes the entire app to silently quit and
relaunch — no error message is shown and no macOS crash report is generated. Models from
the Gemma family load and serve normally; Mistral-family and Qwen models trigger the
reset. After a full clean reinstall, the reset additionally logs the account out and
clears local identity data.

Affected models (all fully downloaded before selecting):
- unsloth/Mistral-Small-3.1-24B-Instruct-2503-GGUF (13 GB) → silent relaunch
- unsloth/Devstral-Small-2505-GGUF (13 GB) → silent relaunch
- Qwen3-14B (9 GB, on fresh install) → silent relaunch + account logged out / identity cleared

Working models on the same machine, same session:
- unsloth/gemma-4-E4B-it-GGUF:Q4_K_M (4.6 GB) — loads and serves
- unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q4_K_M (17 GB) — loaded and served 157 requests

**Steps to reproduce**
1. Open Buzz → Compute → Share compute
2. In the model picker, select a Mistral-family or Qwen model (e.g.
unsloth/Devstral-Small-2505-GGUF) and enable sharing
3. Wait for the model to load — the app quits and relaunches with no error shown
4. (On a clean install) repeat with Qwen3-14B — same relaunch, and the account is
logged out with local identity data cleared

**Expected behavior**
The model either loads and serves, or a visible error explains why it can't
(unsupported architecture, out of memory, etc.). A model-load failure should never
restart the app, and should never touch the identity/keystore or local app data.

**Version and platform**
- Buzz version: 0.5.3 (also confirmed on a freshly downloaded install)
- OS: macOS (Darwin 25.5.0), MacBook Pro M1 Max, 24 GB unified memory

**Logs / additional context**
- No crash reports are generated: ~/Library/Logs/DiagnosticReports has no buzz-desktop
entries from the incident times — the app appears to exit cleanly and relaunch.
- `log stream --predicate 'process CONTAINS[c] "buzz"'` during a reset shows only
WebKit resource-loading noise, zero inference-engine output, then a brand-new
buzz-desktop process (new PID) booting and re-registering with LaunchServices
mid-session — confirming a full app relaunch rather than a window reload.
- npm logs in ~/Library/Application Support/Buzz/node-tools/cache/_logs end with
`exit 0 / info ok` (agent-harness install succeeded; unrelated).
- Models are stored in ~/Library/Caches/huggingface/hub; all affected models were
fully downloaded (sizes verified on disk) before selection, so this is not a
download interruption.
- The clean reinstall that reproduced the identity loss was done by removing
~/Library/Application Support/Buzz, ~/.buzz, ~/Library/Caches/huggingface, and the
app bundle, then installing a freshly downloaded copy.
- Reproducible 100% of the time on this machine (4/4 attempts across two installs).