feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/bun/index.ts

@@ -94,11 +94,11 @@ const ptyHandlers = createPtyHandlers(ptyClient);
 const filesHandlers = createFilesHandlers();
 const settingsHandlers = createSettingsHandlers(settingsDb);
 const agentHandlers = createAgentHandlers(sidecarManager, sessionDb, rpcRef);
-const btmsgHandlers = createBtmsgHandlers(btmsgDb);
-const bttaskHandlers = createBttaskHandlers(bttaskDb);
+const btmsgHandlers = createBtmsgHandlers(btmsgDb, rpcRef);
+const bttaskHandlers = createBttaskHandlers(bttaskDb, rpcRef);
 const searchHandlers = createSearchHandlers(searchDb);
 const pluginHandlers = createPluginHandlers();
-const remoteHandlers = createRemoteHandlers(relayClient);
+const remoteHandlers = createRemoteHandlers(relayClient, settingsDb);
 
 // ── RPC definition ─────────────────────────────────────────────────────────
 
@@ -218,6 +218,17 @@ const rpc = BrowserView.defineRPC<PtyRPCSchema>({
         } catch (err) { console.error("[memora.list]", err); return { memories: [] }; }
       },
 
+      // ── Feature 8: Diagnostics ─────────────────────────────────────────
+      "diagnostics.stats": () => {
+        return {
+          ptyConnected: ptyClient.isConnected,
+          relayConnections: relayClient.listMachines().filter(m => m.status === "connected").length,
+          activeSidecars: sidecarManager.listSessions().filter(s => s.status === "running").length,
+          rpcCallCount: 0, // Placeholder — Electrobun doesn't expose RPC call count
+          droppedEvents: 0,
+        };
+      },
+
       // ── Telemetry ─────────────────────────────────────────────────────
       "telemetry.log": ({ level, message, attributes }) => {
         try { telemetry.log(level, `[frontend] ${message}`, attributes ?? {}); return { ok: true }; }