feat: add append_diagram tool and improve truncation handling (#252)

* feat: add append_diagram tool for truncation continuation

When LLM output hits maxOutputTokens mid-generation, instead of
failing with an error loop, the system now:

1. Detects truncation (missing </root> in XML)
2. Stores partial XML and tells LLM to use new append_diagram tool
3. LLM continues generating from where it stopped
4. Fragments are accumulated until XML is complete
5. Server limits to 5 steps via stepCountIs(5)

Key changes:
- Add append_diagram tool definition in route.ts
- Add append_diagram handler in chat-panel.tsx
- Track continuation mode separately from error mode
- Continuation mode has unlimited retries (not counted against limit)
- Error mode still limited to MAX_AUTO_RETRY_COUNT (1)
- Update system prompts to document append_diagram tool

* fix: show friendly message and yellow badge for truncated output

- Add yellow 'Truncated' badge in UI instead of red 'Error' when XML is incomplete
- Show friendly error message for toolUse.input is invalid errors
- Built on top of append_diagram continuation feature

* refactor: remove debug logs and simplify truncation state

- Remove all debug console.log statements
- Remove isContinuationModeRef, derive from partialXmlRef.current.length > 0

* docs: fix append_diagram instructions for consistency

- Change 'Do NOT include' to 'Do NOT start with' (clearer intent)
- Add <mxCell id="0"> to prohibited start patterns
- Change 'closing tags </root></mxGraphModel>' to just '</root>' (wrapWithMxFile handles the rest)

package.json

@@ -44,6 +44,7 @@
         "clsx": "^2.1.1",
         "js-tiktoken": "^1.0.21",
         "jsdom": "^26.0.0",
+        "jsonrepair": "^3.13.1",
         "lucide-react": "^0.483.0",
         "motion": "^12.23.25",
         "next": "^16.0.7",