Secondary Desktop on the same identity auto-seeds/hosts the builtin Welcome Team, creating duplicate ghost agents that hijack @mentions

# Bug: Secondary Desktop on the same identity auto-seeds/hosts the builtin Welcome Team, creating duplicate ghost agents that hijack @mentions

**Environment**
- Buzz Desktop v0.5.14 (primary: macOS, always-on headless host; secondary: Windows x64)
- Same Nostr identity signed into both Desktops
- Hosted relay

**Summary**
When a second Desktop signs in with an identity whose agents are already hosted on a first Desktop, the second Desktop independently seeds and runs the builtin "Welcome Team" (Fizz/Honey/Bumble) with brand-new keypairs. These duplicate agents join the channel and capture `@mentions` typed on that device, so mentions never reach the real, configured agents on the primary host. The duplicates are unconfigured and only emit `config-nudge` / `BUZZ_ACP_SETUP_PAYLOAD ... unknown variant 'missing_binary'` errors.

**Steps to reproduce**
1. Set up Desktop A (fully configured agent host) with the builtin Welcome Team.
2. Install Desktop B on another machine, sign in with the SAME identity.
3. In a shared channel, `@mention` an agent (e.g. `@Fizz`) from Desktop B.

**Expected**
Desktop B behaves as a client: the mention reaches the existing agent hosted on Desktop A, which replies. No second agent instance is created.

**Actual**
Desktop B seeds its own Fizz/Honey/Bumble (new keys, e.g. a duplicate Fizz alongside the real one), auto-joins them to the channel, and the local unconfigured copy answers the mention with setup-error nudges. The real agents never see the mention.

**Cannot be worked around by hand**
- No "run agents on this device" / disable-hosting toggle exists.
- Deleting `managed(-|_)agents.json` re-seeds on next launch.
- GUI delete is blocked by "Fizz is still assigned to a managed agent" (persona vs managed-agent), and the builtin team (`teams.json`, `is_builtin: true`) re-seeds regardless.
- `buzz channels remove-member` is owner-only and, even when run, the duplicate re-seeds on relaunch.

**Proposed fix (any of)**
1. A secondary Desktop on an identity that already hosts agents should default to **client-only** (no auto-seed, no host).
2. Expose a per-device **"host agents on this device"** toggle (off by default on non-primary devices).
3. Make agent hosting a **single-writer lease** per identity so only one Desktop hosts a given agent, and others act as clients.

**Impact**
Multi-device use (a very common expectation) is currently broken for anyone who hosts agents: the second device cannot reliably talk to its own agents, and it litters channels with duplicate agents and error nudges.

Start by locating the Desktop startup and agent-hosting logic that reads managed_agents.json and teams.json, then reproduce the secondary-login flow described here. Done means a second Desktop does not seed or host duplicate Fizz, Honey, or Bumble agents, and @mentions from it reach the existing primary-hosted agents without setup-error nudges.

Written by the indexing model from the issue text.