chore: add contribution workflow and experiment templates

.gitea/ISSUE_TEMPLATE/idea.md

@@ -0,0 +1,25 @@
+---
+name: Experiment idea
+about: Propose a small experiment or improvement
+title: ""
+---
+
+## The idea
+
+What would you like to explore or improve?
+
+## Who would use it?
+
+Explain the problem or question in concrete terms.
+
+## Smallest useful version
+
+Describe a bounded first version and what is outside its scope.
+
+## How would we check it?
+
+What result would show it works? What assumptions need testing?
+
+## Sources and constraints
+
+Link relevant public references. Note data, license, accessibility, or dependency constraints without sharing private material.