refactor(web): share prompt examples and the shape library with the MCP server

- The three XML examples (swimlanes, two edges, waypoints) that the web
  system prompt, the display_diagram description and the MCP drawing
  guide each had a copy of now live in packages/mcp-server/src/xml-examples.ts
- The shape library group list and reader come from the MCP
  shape-library.ts; getShapeLibrary takes the folder, and the web route
  passes docs/shape-libraries under the working directory as before.
  Only the 30 known library names are read. Error texts differ slightly
- next.config.ts traces docs/shape-libraries/*.md for /api/chat, since
  the read now happens in another module
- Every prompt is byte-for-byte unchanged: the web system prompt for five
  model ids with and without minimal style, and the MCP drawing guide,
  compared before and after; the library list and swimlane example match
  the old tool description text exactly

lib/system-prompts.ts

@@ -1,11 +1,14 @@
 /**
  * System prompts for different AI models
  * Extended prompt is used for models with higher cache token minimums (Opus 4.5, Haiku 4.5)
- *
- * Token counting utilities are in a separate file (token-counter.ts) to avoid
- * WebAssembly issues with Next.js server-side rendering.
  */
 
+import {
+    SWIMLANE_EXAMPLE,
+    TWO_EDGES_EXAMPLE,
+    WAYPOINT_EXAMPLE,
+} from "@/packages/mcp-server/src/xml-examples.ts"
+
 // Default system prompt (~1900 tokens) - works with all models
 export const DEFAULT_SYSTEM_PROMPT = `
 You are an expert diagram creation assistant specializing in draw.io XML generation.
@@ -241,21 +244,7 @@ const EXTENDED_ADDITIONS = `
 
 **Example with swimlanes and edges** (generate ONLY this - no wrapper tags):
 \`\`\`xml
-<mxCell id="lane1" value="Frontend" style="swimlane;" vertex="1" parent="1">
-  <mxGeometry x="40" y="40" width="200" height="200" as="geometry"/>
-</mxCell>
-<mxCell id="step1" value="Step 1" style="rounded=1;" vertex="1" parent="lane1">
-  <mxGeometry x="20" y="60" width="160" height="40" as="geometry"/>
-</mxCell>
-<mxCell id="lane2" value="Backend" style="swimlane;" vertex="1" parent="1">
-  <mxGeometry x="280" y="40" width="200" height="200" as="geometry"/>
-</mxCell>
-<mxCell id="step2" value="Step 2" style="rounded=1;" vertex="1" parent="lane2">
-  <mxGeometry x="20" y="60" width="160" height="40" as="geometry"/>
-</mxCell>
-<mxCell id="edge1" style="edgeStyle=orthogonalEdgeStyle;endArrow=classic;" edge="1" parent="1" source="step1" target="step2">
-  <mxGeometry relative="1" as="geometry"/>
-</mxCell>
+${SWIMLANE_EXAMPLE}
 \`\`\`
 
 ### append_diagram Details
@@ -318,12 +307,7 @@ If cell_id not found, check "Current diagram XML" for correct IDs. Use display_d
 
 ### Two edges between same nodes (CORRECT - no overlap):
 \`\`\`xml
-<mxCell id="e1" value="A to B" style="edgeStyle=orthogonalEdgeStyle;exitX=1;exitY=0.3;entryX=0;entryY=0.3;endArrow=classic;" edge="1" parent="1" source="a" target="b">
-  <mxGeometry relative="1" as="geometry"/>
-</mxCell>
-<mxCell id="e2" value="B to A" style="edgeStyle=orthogonalEdgeStyle;exitX=0;exitY=0.7;entryX=1;entryY=0.7;endArrow=classic;" edge="1" parent="1" source="b" target="a">
-  <mxGeometry relative="1" as="geometry"/>
-</mxCell>
+${TWO_EDGES_EXAMPLE}
 \`\`\`
 
 ### Edge with single waypoint (simple detour):
@@ -342,14 +326,7 @@ If cell_id not found, check "Current diagram XML" for correct IDs. Use display_d
 **WRONG:** Direct diagonal line crosses over Develop
 **CORRECT:** Route around the OUTSIDE (go right first, then up)
 \`\`\`xml
-<mxCell id="hotfix_to_main" style="edgeStyle=orthogonalEdgeStyle;exitX=0.5;exitY=0;entryX=1;entryY=0.5;endArrow=classic;" edge="1" parent="1" source="hotfix" target="main">
-  <mxGeometry relative="1" as="geometry">
-    <Array as="points">
-      <mxPoint x="750" y="80"/>
-      <mxPoint x="750" y="150"/>
-    </Array>
-  </mxGeometry>
-</mxCell>
+${WAYPOINT_EXAMPLE}
 \`\`\`
 This routes the edge to the RIGHT of all shapes (x=750), then enters Main from the right side.