chore: add contribution workflow and experiment templates

docs/experiment-template.md

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+# Experiment name
+
+Replace this template with concrete details before publishing an experiment.
+
+## Purpose
+
+What question does this explore or what problem does it solve? Who is it for?
+
+## Status
+
+Idea / prototype / usable / archived. State the main limitation and what works today.
+
+## Try it
+
+List prerequisites and exact dependency, setup, and run commands. Include synthetic sample input and expected output. Explain how to stop it and remove any temporary data. If there is no executable code, say how to inspect or use the artifact.
+
+## Checks
+
+Record reproducible checks and actual results. Separate automated tests, manual checks, and checks not yet run. For visual work, document relevant screen sizes and accessibility checks.
+
+## Data and privacy
+
+Explain where sample data comes from and whether anything leaves the local machine. Never include credentials or private records. Document configuration using placeholder values only.
+
+## Limitations and next steps
+
+List known gaps and the smallest useful next step. Avoid implying production readiness.
+
+## Sources and license
+
+Credit reused code, data, images, and other material with source links and applicable licenses. Retain required notices.