fix: USE_MAGICK build flag - use define instead of bun:bundle

The bun:bundle module only exists at bundle time, not when running source
directly (dev/tests). This caused CI failures with:
  error: Cannot find package 'bundle'

Fix: Use Bun.build's define option to set __USE_MAGICK__ as a compile-time
constant. At dev/test time it's undefined (defaults to false/sharp).
At build time it's set based on USE_MAGICK env var.

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

build.js

@@ -39,6 +39,7 @@ await Bun.build({
   define: {
     LETTA_VERSION: JSON.stringify(version),
     BUILD_TIME: JSON.stringify(new Date().toISOString()),
+    __USE_MAGICK__: useMagick ? "true" : "false",
   },
   // Load text files as strings (for markdown, etc.)
   loader: {