feat: add conversation management, persistence, and mobile app spec

Major enhancements to voice assistant with conversation persistence
and mobile app planning:

- Add conversation management with multiple active conversations
- Implement conversation persistence (save/load from disk)
- Add conversation selector UI for switching between conversations
- Enhance agent prompt with per-conversation tmux sessions
- Add mobile app technical specification (React Native + Expo)
- Improve UI with active processes view and agent stream
- Add ACP (Agent Control Protocol) server implementation

Mobile spec details:
- React Native with Expo SDK 54+
- NativeWind v5 for styling
- Share Zod schemas/types from server
- Android in-call experience (InCallManager)
- Direct WebSocket connection via Tailscale
- Same audio quality as web prototype

🤖 Generated with [Claude Code](https://claude.com/claude-code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

packages/voice-assistant/src/server/persistence.ts

@@ -0,0 +1,154 @@
+import { readFile, writeFile, readdir, unlink, mkdir, stat } from "fs/promises";
+import { join } from "path";
+import type { ModelMessage } from "@ai-sdk/provider-utils";
+import { standardizePrompt } from "ai/internal";
+
+const CONVERSATIONS_DIR = join(process.cwd(), "conversations");
+
+interface ConversationMetadata {
+  id: string;
+  lastUpdated: Date;
+  messageCount: number;
+}
+
+interface ConversationData {
+  conversationId: string;
+  lastUpdated: string;
+  messageCount: number;
+  messages: ModelMessage[];
+}
+
+/**
+ * Ensure conversations directory exists
+ */
+async function ensureConversationsDir(): Promise<void> {
+  await mkdir(CONVERSATIONS_DIR, { recursive: true });
+}
+
+/**
+ * Save conversation to disk
+ */
+export async function saveConversation(
+  conversationId: string,
+  messages: ModelMessage[]
+): Promise<void> {
+  try {
+    await ensureConversationsDir();
+
+    const filepath = join(CONVERSATIONS_DIR, `${conversationId}.json`);
+    const data: ConversationData = {
+      conversationId,
+      lastUpdated: new Date().toISOString(),
+      messageCount: messages.length,
+      messages,
+    };
+
+    await writeFile(filepath, JSON.stringify(data, null, 2), "utf-8");
+    console.log(
+      `[Persistence] Saved conversation ${conversationId} (${messages.length} messages)`
+    );
+  } catch (error) {
+    console.error(
+      `[Persistence] Failed to save conversation ${conversationId}:`,
+      error
+    );
+    throw error;
+  }
+}
+
+/**
+ * Load conversation from disk
+ * Returns null if conversation doesn't exist or fails to parse
+ */
+export async function loadConversation(
+  conversationId: string
+): Promise<ModelMessage[] | null> {
+  try {
+    const filepath = join(CONVERSATIONS_DIR, `${conversationId}.json`);
+
+    // Check if file exists
+    try {
+      await stat(filepath);
+    } catch {
+      console.log(`[Persistence] Conversation ${conversationId} not found`);
+      return null;
+    }
+
+    // Read and parse file
+    const fileContent = await readFile(filepath, "utf-8");
+    const data: ConversationData = JSON.parse(fileContent);
+
+    // Validate and standardize messages using AI SDK utility
+    const result = await standardizePrompt({
+      prompt: data.messages,
+    });
+
+    console.log(
+      `[Persistence] Loaded conversation ${conversationId} (${data.messageCount} messages)`
+    );
+
+    return result.messages as ModelMessage[];
+  } catch (error) {
+    console.error(
+      `[Persistence] Failed to load conversation ${conversationId}:`,
+      error
+    );
+    return null;
+  }
+}
+
+/**
+ * List all conversations with metadata
+ */
+export async function listConversations(): Promise<ConversationMetadata[]> {
+  try {
+    await ensureConversationsDir();
+
+    const files = await readdir(CONVERSATIONS_DIR);
+    const jsonFiles = files.filter((f) => f.endsWith(".json"));
+
+    const conversations: ConversationMetadata[] = [];
+
+    for (const file of jsonFiles) {
+      try {
+        const filepath = join(CONVERSATIONS_DIR, file);
+        const fileContent = await readFile(filepath, "utf-8");
+        const data: ConversationData = JSON.parse(fileContent);
+
+        conversations.push({
+          id: data.conversationId,
+          lastUpdated: new Date(data.lastUpdated),
+          messageCount: data.messageCount,
+        });
+      } catch (error) {
+        console.error(`[Persistence] Failed to read conversation ${file}:`, error);
+        // Skip invalid files
+      }
+    }
+
+    // Sort by lastUpdated descending
+    conversations.sort((a, b) => b.lastUpdated.getTime() - a.lastUpdated.getTime());
+
+    return conversations;
+  } catch (error) {
+    console.error("[Persistence] Failed to list conversations:", error);
+    return [];
+  }
+}
+
+/**
+ * Delete conversation from disk
+ */
+export async function deleteConversation(conversationId: string): Promise<void> {
+  try {
+    const filepath = join(CONVERSATIONS_DIR, `${conversationId}.json`);
+    await unlink(filepath);
+    console.log(`[Persistence] Deleted conversation ${conversationId}`);
+  } catch (error) {
+    console.error(
+      `[Persistence] Failed to delete conversation ${conversationId}:`,
+      error
+    );
+    throw error;
+  }
+}