Snapshot import has no update path: reimport duplicates, and the team it created cannot be deleted

# Snapshot import has no update path: reimport duplicates, and the team it created cannot be deleted

## Summary

Importing a `.team.json` snapshot is a one-way operation. There is no way to
update an imported team in place, and no practical way to remove one, so any
change to the source of a snapshot leaves the operator stuck between duplicates
and a manual 27-step cleanup.

Two behaviours combine into the dead end:

1. **Import always mints new agents.** `confirmAgentSnapshotImport` is
documented as importing "as a brand-new agent with fresh keys", and snapshots
deliberately strip every lineage field (`persona_id`, `persona_source_version`,
`source_team`, `source_team_persona_slug`). Nothing on the wire identifies two
snapshots as versions of the same agent, so reimport can only duplicate.

2. **Delete refuses while members exist.** `delete_team_with_cascade` cascades
personas that came from a `source_dir`, but a JSON-imported team has none, so
`agents_referencing_team` is non-empty and the delete hard-fails.

## Steps to reproduce

1. Author a `buzz-team-snapshot v1` file with 27 members.
2. Import it in Desktop. 27 agents are created and joined to a new team.
3. Change one member's system prompt at the source and reimport.
→ 54 agents. No prompt was updated.
4. Try to delete the team to start clean.

```
Cannot delete team "5ff0eeb8-07c6-4c45-9ba6-4cbf57fbbc89": 27 agent(s) still
reference it (Blender Addon Engineer, Economy Designer, Game Audio Engineer,
... 24 more). Delete or reconfigure them first.
```

The only supported path is deleting 27 agents individually in the UI. There is
no bulk delete, and no `buzz agents delete` in the CLI. Editing
`managed-agents.json` by hand would orphan the relay-side kind:30175 persona
heads that `delete_team_with_cascade` documents as needing NIP-09 tombstones.

## Impact

This scales with team size, and team snapshots exist precisely to make large
teams importable in one step. A 27-agent team is 27 manual deletions per
iteration. It also makes snapshots unusable as a distribution format for any
library that changes over time, since consumers can receive a v1 but never a
v2.

## Proposed fixes

Three, listed smallest first. They are independent.

**1. `cascade_members` on team delete.** An explicit opt-in flag on the delete
command that removes referencing agents and enqueues their tombstones, reusing
the cascade path that already exists for `source_dir` personas. Keeps the
current refusal as the default. This alone unblocks the reimport workflow.

**2. Bulk delete in the UI, and a `buzz agents delete` subcommand.** Precedent:
[#2976](https://github.com/block/buzz/issues/2976) asks for `buzz repos delete`
for the same reason.

**3. Optional `sourceId` on the snapshot, enabling idempotent import.** A
stable, caller-supplied identifier that survives export. On import, a matching
`sourceId` offers update-in-place (definition and profile only, preserving keys,
memory, and channel membership) instead of minting a duplicate. This is the root
fix; 1 and 2 are workarounds for its absence.

`sourceId` is deliberately not a lineage field of the kind the format already
excludes on privacy grounds: it identifies the *source artifact*, not the
exporting machine, relay, or account, and it is opt-in per snapshot. Adding it
to v1 as an optional field is backward compatible in both directions, but it is
a wire-format change and worth agreeing on before anyone writes it.

## Related

- [#2973](https://github.com/block/buzz/issues/2973) — stale duplicate agents that cannot be archived or removed. Same removal gap, reached from a different direction.
- [#2648](https://github.com/block/buzz/issues/2648) — duplicate agent identities from a second Desktop install.
- [#1725](https://github.com/block/buzz/issues/1725) — orphaned duplicate agent definitions.

## Environment

- Buzz Desktop (macOS), packaged build
- Snapshots authored externally as `buzz-team-snapshot v1`

## Offer

Happy to open a PR for (1), which looks self-contained in
`desktop/src-tauri/src/managed_agents/teams.rs` plus the command wrapper and a
confirmation in the delete dialog. Would rather agree the shape of (3) here
before touching the format.

Start in desktop/src-tauri/src/managed_agents/teams.rs at delete_team_with_cascade, then trace the command wrapper and delete dialog mentioned in the issue. First agree whether the contribution targets cascade_members, bulk deletion, or sourceId-based reimport; done means the selected workflow is defined without breaking the current default refusal.

Written by the indexing model from the issue text.