feat(chats): add managed projectless chat sessions

Create projectless chat sessions under a managed, date-scoped Chats directory and clean abandoned or deleted session folders.

Add Chats to sidebar state, startup cache, shared context, and Electron Mini Chat while keeping VS Code project-only. Resolve managed chat directories to one server-side memory owner and document the runtime contracts.

packages/ui/src/sync/session-actions.test.ts

@@ -125,6 +125,7 @@ mock.module("@/lib/opencode/client", () => ({
       return mockScopedClient
     },
     getDirectory: () => "/test/project",
+    getFilesystemHome: mock(async () => "/home/test"),
     getSdkClient: () => mockSdk,
     replyToPermission: mock((requestId: string, reply: string, options?: { directory?: string | null }) => {
       replyCalls.push({ method: "permission.reply", params: { requestID: requestId, reply, directory: options?.directory } })