chore: add contribution workflow and experiment templates

.gitea/ISSUE_TEMPLATE/bug.md

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+---
+name: Bug report
+about: Report a reproducible problem in an existing experiment
+title: ""
+---
+
+## Experiment and version
+
+Link the affected directory and identify the commit or version.
+
+## What happened?
+
+Describe the observed result and what you expected.
+
+## Reproduce it
+
+1.
+2.
+3.
+
+## Environment
+
+Operating system, browser or runtime version, and relevant configuration. Use placeholder values for private configuration.
+
+## Evidence
+
+Include a minimal example or redacted screenshot/log. Do not post credentials, tokens, personal records, or private URLs.
+
+## Checks already tried
+
+State what you tried and what happened.

.gitea/ISSUE_TEMPLATE/idea.md

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+---
+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.