refactor(sdk): unify message types with pi-ai source of truth

Replace hand-written message/content types in @multica/sdk with
`import type` from @mariozechner/pi-ai and @mariozechner/pi-agent-core.
This ensures compile-time correctness and eliminates type drift between
backend and frontend (e.g. "tool_use" vs "toolCall", "tool_result" vs
"toolResult").

- stream.ts: re-export TextContent, ThinkingContent, ToolCall,
  ImageContent from pi-ai; use AgentEvent from pi-agent-core
- rpc.ts: AgentMessageItem = pi-ai Message (no more manual mirroring)
- connection-store.ts: use SDK types instead of inline hand-written ones
- ContentBlock now includes ImageContent to match backend reality

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/sdk/package.json

@@ -17,6 +17,8 @@
     "uuid": "^13.0.0"
   },
   "devDependencies": {
+    "@mariozechner/pi-agent-core": "^0.50.3",
+    "@mariozechner/pi-ai": "^0.50.3",
     "@types/uuid": "^11.0.0",
     "typescript": "^5.9.3"
   }