chore: setup scratch file directory

Scratch file directory is a place where we can put temporary scripts for
development, so it is git-ignored and some lint checks are disabled on it.

Author: cjumel

.gitignore

@@ -1,21 +1,30 @@
-# Python-generated files
-.idea/
+# Environment variables
+.env
+
+# Reserved directories
+data/
+scratch/
+
+# Python artefacts
 .ipynb_checkpoints/
 .venv/
-.vscode/
 __pycache__/
 build/
 dist/
 wheels/
 .coverage
-.env
 *.egg-info
 *.py[oc]
 
-# MacOS stuff
-.DS_Store
+# IDE artefacts
+.cursor/
+.idea/
+.vscode/
 
-# Coding agents stuff
+# Coding agents artefacts
+.claude/
+.opencode/
 .rtk/
 
-playground.py
+# System artefacts
+.DS_Store

.rumdl.toml

@@ -1,4 +1,5 @@
 [global]
+exclude = ["scratch/**"]
 line-length = 100
 
 [MD013]

AGENTS.md

@@ -37,7 +37,11 @@ When adding or changing a public API capability, update the relevant pieces toge
 
 ## Validation
 
-Before opening a PR, run the CI checks: `make lint typecheck test`.
+After having done some changes or before opening a PR, run the CI checks:
+`make lint typecheck test`.
+
+Never run the underlying checks without using these `make` commands (e.g. with `uv run ruff ...`),
+as these kind of checks won't take into account some settings (like the `exclude` list).
 
 ## Non-Goals
 

pyproject.toml

@@ -62,6 +62,7 @@ target-version = "py310"
 unsafe-fixes = true
 
 [tool.ruff.lint]
+exclude = ["scratch/"]
 explicit-preview-rules = true
 extend-ignore = ["D107"]
 preview = true