Re-document and Nix integration with IDEs.

CONTRIBUTING.md

@@ -0,0 +1,39 @@
+# Contributing
+
+## Nix + IDE Integration
+
+If your IDE reports that a file doesn't belong to the project or that files cannot be found, the CMake cache is likely
+missing the Nix-provided paths.
+
+This may happen when the IDE runs CMake on its own, outside the Nix environment, leaving the required paths empty.
+
+#### Preferred fix: launch the IDE from inside the Nix dev shell
+
+This makes the IDE inherit required environment variables directly, so every CMake reload it triggers has the right
+environment, not just a one-off cache snapshot.
+
+Example:
+```bash
+nix develop -c ...
+```
+
+Or, if you're using CLion, there's already a shortcut for that:
+```bash
+just clion
+```
+
+#### Alternative: regenerate the cache, then reload without resetting it
+
+If you don't want to launch the IDE from inside the Nix dev shell, you can try to reuse an existing `CMakeCache.txt` on
+reload instead of always re-running `cmake` with its own environment.
+
+To do that, you first need to regenerate the cache. This can be done by running:
+```bash
+nix develop -c just configure
+```
+
+Then reload the CMake project without resetting the cache (resetting it would wipe the paths you just wrote):
+
+> JetBrains: **View → Tool Windows → CMake** → **Reload** ↺
+
+This won't help on an IDE that always re-invokes `cmake` with its own (non-Nix) environment on reload.

justfile

@@ -17,6 +17,10 @@ rebuild: clean build
 nix:
     nix develop
 
+# Launch CLion inside the Nix dev shell so it inherits required variables and fully integrates with Nix.
+clion:
+    nix develop -c "$HOME/.local/share/JetBrains/Toolbox/apps/clion/bin/clion.sh" . >/dev/null 2>&1 &
+
 prettify:
     nix shell nixpkgs#clang-tools -c clang-format -i src/**.cpp src/**.h