docs(rules): add development guidelines

Amolith created 

The new `.rules` file outlines a standardized process for LLMs to follow after
completing requests. This includes checking project diagnostics and running
automated checks to ensure code quality and consistency.

Change summary

.rules | 10 ++++++++++
1 file changed, 10 insertions(+)

Detailed changes

.rules 🔗 

@@ -0,0 +1,10 @@
+<!--
+SPDX-FileCopyrightText: Amolith <amolith@secluded.site>
+
+SPDX-License-Identifier: CC0-1.0
+-->
+
+- After completing requests, check the project diagnostics and resolve each
+  issue, one by one, according to your rules.
+- Only after you've resolved all diagnostics should you run `just` in the
+  terminal, if available, to perform further checks.