fix: use ACP protocol stopReason for proper agent completion detection

Replace unreliable 2-second timeout heuristic with proper protocol-based completion detection using PromptResponse.stopReason from ACP SDK.

Changes:
- Capture and handle PromptResponse.stopReason in sendPrompt()
- Set agent status based on stop reason: end_turn → completed, refusal → failed, cancelled → ready
- Remove 2-second timeout completion heuristic (was causing drift)
- Update SessionStateMessageSchema to match AgentInfo null types

This ensures agent completion is detected via the protocol spec, not manual polling/timing.

Generated with [Claude Code](https://claude.ai/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/messages.ts

@@ -166,14 +166,14 @@ export const SessionStateMessageSchema = z.object({
         status: z.string(),
         createdAt: z.date(),
         type: z.literal("claude"),
-        sessionId: z.string().optional(),
-        error: z.string().optional(),
-        currentModeId: z.string().optional(),
+        sessionId: z.string().nullable(),
+        error: z.string().nullable(),
+        currentModeId: z.string().nullable(),
         availableModes: z.array(z.object({
           id: z.string(),
           name: z.string(),
           description: z.string().nullable().optional(),
-        })).optional(),
+        })).nullable(),
       })
     ),
     commands: z.array(