Add project documentation and repository structure

CONTRIBUTING.md

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+# Contributing to Hudra
+
+Hudra is currently an early-stage project.
+
+## Development principles
+
+Changes should be:
+
+- focused
+- easy to understand
+- independently testable
+- backwards compatible where practical
+
+## Before making a change
+
+1. Create or identify an issue.
+2. Describe the problem being solved.
+3. Define clear acceptance criteria.
+4. Make the smallest practical change.
+5. Test against the stable lesson examples.
+6. Confirm existing presentation behaviour still works.
+
+## Commit messages
+
+Use short, descriptive commit messages, for example:
+
+- Fix audience zoom reset
+- Add presenter keyboard shortcut
+- Improve image-only slide sizing
+- Update installation documentation
+
+## Releases
+
+Stable releases should include:
+
+- an updated manifest version
+- changelog entry
+- Git commit
+- Git tag
+- release ZIP