chore: add contribution workflow and experiment templates

.gitea/PULL_REQUEST_TEMPLATE.md

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+## Purpose
+
+Explain the problem or question this change addresses. Link a related issue if one exists.
+
+## Changes
+
+-
+
+## Verification
+
+List exact commands or manual checks and their results. State what was not tested. Add screenshots for visual changes where useful.
+
+## Review checklist
+
+- [ ] The change is focused and documented.
+- [ ] Setup and usage instructions match the actual files.
+- [ ] New experiments are listed in docs/experiments.md.
+- [ ] Sources and required license notices are included.
+- [ ] Files, screenshots, and staged changes contain no secrets or private material.
+- [ ] Limitations and unrun checks are stated clearly.
+
+## Risks or follow-up
+
+Describe remaining gaps, compatibility concerns, or rollback steps. Mark checklist items not applicable with an explanation.