fix(excalidraw): resolve canvas module path and add canonical file location convention

Fix convert.mjs to resolve canvas from .export-runtime via createRequire
instead of bare import (which resolves relative to script location, not CWD).
Add File Location Convention section to SKILL.md — diagrams save .excalidraw
source alongside PNGs in the project's image directory for easy re-export.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/compound-engineering/skills/excalidraw-png-export/SKILL.md

@@ -23,6 +23,20 @@ This creates a `.export-runtime` directory inside `scripts/` with the Node.js de
 
 The Excalidraw MCP server must be configured. Verify availability by checking for `mcp__excalidraw__create_view` and `mcp__excalidraw__read_checkpoint` tools.
 
+## File Location Convention
+
+Save diagram source files alongside their PNG exports in the project's image directory. This enables re-exporting diagrams when content or styling changes.
+
+**Standard pattern:**
+```
+docs/images/my-diagram.excalidraw    # source (commit this)
+docs/images/my-diagram.png           # rendered output (commit this)
+```
+
+**When updating an existing diagram**, look for a `.excalidraw` file next to the PNG. If one exists, edit it and re-export rather than rebuilding from scratch.
+
+**Temporary files** (raw checkpoint JSON) go in `/tmp/excalidraw-export/` and are discarded after conversion.
+
 ## Workflow
 
 ### Step 1: Design the Diagram Elements
@@ -63,11 +77,11 @@ Use the `convert.mjs` script to transform raw MCP checkpoint JSON into a valid `
 - Expands `label` properties on shapes/arrows into proper bound text elements
 
 ```bash
-# Save checkpoint JSON to a file first, then convert:
-node <skill-path>/scripts/convert.mjs <input.json> <output.excalidraw>
+# Save checkpoint JSON to a temp file, then convert to the project's image directory:
+node <skill-path>/scripts/convert.mjs /tmp/excalidraw-export/raw.json docs/images/my-diagram.excalidraw
 ```
 
-The input JSON should be the raw checkpoint data from `mcp__excalidraw__read_checkpoint` (the `{"elements": [...]}` object).
+The input JSON should be the raw checkpoint data from `mcp__excalidraw__read_checkpoint` (the `{"elements": [...]}` object). The output `.excalidraw` file goes in the project's image directory (see File Location Convention above).
 
 **For batch exports**: Write each checkpoint to a separate raw JSON file, then convert each one:
 ```bash
@@ -92,7 +106,7 @@ Bound text (labels on shapes/arrows) needs: `containerId: "<parent-id>"`, `textA
 Run the export script. Determine the runtime path relative to this skill's scripts directory:
 
 ```bash
-cd <skill-path>/scripts/.export-runtime && node <skill-path>/scripts/export_png.mjs /tmp/excalidraw-export/diagram.excalidraw /tmp/excalidraw-export/output.png
+cd <skill-path>/scripts/.export-runtime && node <skill-path>/scripts/export_png.mjs docs/images/my-diagram.excalidraw docs/images/my-diagram.png
 ```
 
 The script:
@@ -110,7 +124,7 @@ The script prints the output path on success. Verify the result with `file <outp
 Run the validation script on the `.excalidraw` file to catch spatial issues:
 
 ```bash
-node <skill-path>/scripts/validate.mjs /tmp/excalidraw-export/diagram.excalidraw
+node <skill-path>/scripts/validate.mjs docs/images/my-diagram.excalidraw
 ```
 
 Then read the exported PNG back using the Read tool to visually inspect:

plugins/compound-engineering/skills/excalidraw-png-export/scripts/convert.mjs

@@ -4,13 +4,20 @@
  * Filters pseudo-elements, adds required defaults, expands labels into bound text.
  */
 import { readFileSync, writeFileSync } from 'fs';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+import { createRequire } from 'module';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const runtimeRequire = createRequire(join(__dirname, '.export-runtime', 'package.json'));
 
 // Canvas-based text measurement with graceful fallback to heuristic.
 // Excalidraw renders with Virgil (hand-drawn font); system sans-serif
 // is a reasonable proxy. The 1.1x multiplier accounts for Virgil being wider.
 let measureText;
 try {
-  const { createCanvas } = await import('canvas');
+  const canvas = runtimeRequire('canvas');
+  const { createCanvas } = canvas;
   const cvs = createCanvas(1, 1);
   const ctx = cvs.getContext('2d');
   measureText = (text, fontSize) => {