docs: keep dev builds out of the stable app

CLAUDE.md

@@ -12,3 +12,16 @@ Keep everything short — replies, explanations, comments, docs.
 - Body only when something genuinely cannot be inferred from the diff.
 - Never add `Co-Authored-By`, "Generated with" or any other AI attribution to commits or PR descriptions.
 - Exception: the commits in `build/upstream` that `patches/` is exported from. Their subjects become the patch file names, so they stay plain and descriptive.
+
+## Dev builds
+
+Never build over an installed Modrinth Enhanced. The stable app on this machine is installed
+separately, from the AUR, and stays untouched.
+
+- `DEV=1 scripts/build.sh` builds "Modrinth Enhanced (dev)". Its bundle identifier is
+  `ModrinthAppDev`, so it has a data directory of its own and shares no instances, accounts or
+  settings with the stable app, and it has no updater.
+- Install it beside the stable app, never over it: `~/.local/bin/Modrinth.Enhanced.dev.AppImage`
+  with a desktop entry of its own.
+- `FAST=1` cuts the build to about a third of the time, at the cost of a few megabytes. Use it for
+  anything that is not meant to be release-like: `DEV=1 FAST=1 scripts/build.sh`.