feat(site-builder): stamp git sha + date into the editor bundle

package.json's version is hand-maintained and never changes between
builds, so a bug report can't identify which bundle produced it.
Vite's `define` injects __EDITOR_BUILD__ (short git SHA + build date)
at compile time; editorBuild() in build-stamp.ts is the only safe way
to read it, falling back to 'dev' since vitest does not apply Vite's
`define` and the identifier is otherwise undeclared. The execSync
call falls back to 'nogit' when building outside a git checkout
(release tarballs), verified by building from a directory with no
git ancestry at all.

Also wires editorBuild() into a startup console.log in main.tsx --
without any reference to it, Vite tree-shakes the unused module out
of the bundle entirely and __EDITOR_BUILD__ never gets substituted,
silently leaving every bug report saying 'dev'. Task 19 will add the
real call site when it assembles the report payload.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

craft/vite.config.ts

@@ -1,6 +1,20 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 import path from 'path'
+import { execSync } from 'child_process'
+
+/** Short git SHA + build date, injected as __EDITOR_BUILD__ so an issue
+ *  report identifies exactly which bundle produced it. package.json's
+ *  version is hand-maintained and never changes between builds. */
+const editorBuild = (() => {
+  let sha = 'nogit'
+  try {
+    sha = execSync('git rev-parse --short HEAD', { cwd: __dirname }).toString().trim()
+  } catch {
+    // Building outside a git checkout (release tarball) -- keep 'nogit'.
+  }
+  return `${sha}-${new Date().toISOString().slice(0, 10)}`
+})()
 
 export default defineConfig({
   plugins: [react()],
@@ -10,6 +24,9 @@ export default defineConfig({
       '@': path.resolve(__dirname, './src'),
     },
   },
+  define: {
+    __EDITOR_BUILD__: JSON.stringify(editorBuild),
+  },
   build: {
     outDir: 'dist',
     rollupOptions: {