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/src/utils/build-stamp.ts

@@ -0,0 +1,10 @@
+/**
+ * Reads the compile-time `__EDITOR_BUILD__` define.
+ *
+ * vitest does not apply Vite's `define`, and `npm run dev` in a non-git
+ * checkout may not either, so every read goes through here. `typeof` on an
+ * undeclared identifier is safe in JS -- it does not throw.
+ */
+export function editorBuild(): string {
+  return typeof __EDITOR_BUILD__ !== 'undefined' ? __EDITOR_BUILD__ : 'dev';
+}