feat(desktop): add persistent Bestie experience (#7223)

## Summary

- add a persistent, community-scoped Bestie designation for local
managed agents
- surface the designated agent in the sidebar, agent library, profile
actions, message toolbar, and draggable floating shortcut
- bloom the floating shortcut into a lightweight compact composer that
reuses the normal DM timeline, reactions, presence, and send behavior
- support message handoff with a bounded snapshot and a full Buzz thread
link so the agent can retrieve the complete conversation

## UX details

- the floating avatar and expanded panel stay above app chrome and drag
as one aligned surface
- closing the expanded panel returns it to its top-right anchor
- each mini-composer opening starts visually fresh while messages sent
during that opening remain conversational
- the designated Bestie's duplicate DM entry is hidden from the regular
DM list
- Bestie actions are suppressed inside the mini timeline to avoid
recursive handoff

## Reliability and maintainability

- preserve existing retention database paths across upgrades
- serialize assignment and deletion, clearing matching assignments
across community scopes before an agent is removed
- fence async conversation resolution against workspace and assignment
changes
- validate stale assignments against existing local agents before hiding
DMs
- share lightweight assignment state across agent cards and keep
protected-feature behavior out of the shared timeline API

## Testing

- `pnpm --dir desktop test` — 5,886 tests passed
- `pnpm --dir desktop exec tsc --noEmit`
- `pnpm --dir desktop exec biome check ...`
- `cargo clippy --manifest-path desktop/src-tauri/Cargo.toml
--all-targets -- -D warnings`
- focused native retention and Bestie assignment/command tests — 34
passed
- `VITE_BUZZ_BESTIE=1 pnpm --dir desktop build:e2e`
- `VITE_BUZZ_BESTIE=1 pnpm --dir desktop exec playwright test
--project=smoke tests/e2e/bestie.spec.ts`
- pre-commit and differential pre-push hooks

## Rollout

The UI remains gated by the `bestie` build feature. Screenshots covering
setup, empty, assigned, floating, and message-handoff states are
included in the PR discussion.

---------

Signed-off-by: Arjun Mahanti <arjun@squareup.com>
Co-authored-by: Codex <noreply@openai.com>

desktop/src/testing/e2eBridge.ts

@@ -2475,6 +2475,7 @@ function resetMockRelayAgents(config?: E2eConfig) {
 
 function resetMockManagedAgents(config?: E2eConfig) {
   mockManagedAgents = [];
+  mockBestieAssignment = null;
   mockManagedAgentRuntimes = (config?.mock?.managedAgentRuntimes ?? []).map(
     (seed) => ({
       pubkey: seed.pubkey,
@@ -3225,6 +3226,9 @@ let mockClosedChannelLiveSubscription = false;
 const realSockets = new Map<number, WebSocket>();
 let mockManagedAgents: MockManagedAgent[] = [];
 let mockManagedAgentRuntimes: MockManagedAgentRuntimeRow[] = [];
+let mockBestieAssignment: {
+  agent_pubkey: string;
+} | null = null;
 
 // Mutable `save_subscriptions` table mirror — TEST-ONLY.
 //
@@ -9694,6 +9698,9 @@ async function handleDeleteManagedAgent(args: {
   mockManagedAgents = mockManagedAgents.filter(
     (candidate) => candidate.pubkey !== args.pubkey,
   );
+  if (mockBestieAssignment?.agent_pubkey === args.pubkey) {
+    mockBestieAssignment = null;
+  }
   syncMockRelayAgentsFromManagedAgents();
 }
 
@@ -13734,6 +13741,32 @@ export function maybeInstallE2eTauriMocks() {
         );
       case "list_managed_agent_runtimes":
         return mockManagedAgentRuntimes.map((row) => ({ ...row }));
+      case "get_bestie_assignment":
+        return mockBestieAssignment ? { ...mockBestieAssignment } : null;
+      case "assign_bestie": {
+        const agentPubkey = (payload as { agentPubkey: string }).agentPubkey;
+        const agent = getMockManagedAgent(agentPubkey);
+        if (agent.backend.type !== "local") {
+          throw new Error("only a local managed agent can be your Bestie");
+        }
+        mockBestieAssignment = {
+          agent_pubkey: agent.pubkey,
+        };
+        return { ...mockBestieAssignment };
+      }
+      case "clear_bestie_assignment":
+        mockBestieAssignment = null;
+        return undefined;
+      case "resolve_bestie_conversation": {
+        if (!mockBestieAssignment) {
+          throw new Error("choose an agent before opening Bestie");
+        }
+        const channel = await handleOpenDm(
+          { pubkeys: [mockBestieAssignment.agent_pubkey] },
+          activeConfig,
+        );
+        return channel;
+      }
       case "start_managed_agent_runtime":
         return handleManagedAgentRuntimeAction(
           "start",