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/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) => {