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)

lib/system-prompts.ts

@@ -42,11 +42,18 @@ description: Edit specific parts of the EXISTING diagram. Use this when making s
 parameters: {
   edits: Array<{search: string, replace: string}>
 }
+---Tool3---
+tool name: append_diagram
+description: Continue generating diagram XML when display_diagram was truncated due to output length limits. Only use this after display_diagram truncation.
+parameters: {
+  xml: string  // Continuation fragment (NO wrapper tags like <mxGraphModel> or <root>)
+}
 ---End of tools---
 
 IMPORTANT: Choose the right tool:
 - Use display_diagram for: Creating new diagrams, major restructuring, or when the current diagram XML is empty
 - Use edit_diagram for: Small modifications, adding/removing elements, changing text/colors, repositioning items
+- Use append_diagram for: ONLY when display_diagram was truncated due to output length - continue generating from where you stopped
 
 Core capabilities:
 - Generate valid, well-formed XML strings for draw.io diagrams
@@ -174,6 +181,18 @@ const EXTENDED_ADDITIONS = `
 </root>
 \`\`\`
 
+### append_diagram Details
+
+**WHEN TO USE:** Only call this tool when display_diagram output was truncated (you'll see an error message about truncation).
+
+**CRITICAL RULES:**
+1. Do NOT start with <mxGraphModel>, <root>, or <mxCell id="0"> - they already exist in the partial
+2. Continue from EXACTLY where your previous output stopped
+3. End with the closing </root> tag to complete the diagram
+4. If still truncated, call append_diagram again with the next fragment
+
+**Example:** If previous output ended with \`<mxCell id="x" style="rounded=1\`, continue with \`;" vertex="1">...\` and complete the remaining elements.
+
 ### edit_diagram Details
 
 **CRITICAL RULES:**