docs(contributing): add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+We're glad you're interested in contributing to Logos Execution Zone!
+
+This document describes the guidelines for contributing to the project. We will be updating it as we grow and we figure out what works best for us.
+
+If you have any questions, come say hi to our [Discord](https://discord.gg/tGJwgGrSPN)!
+
+## Commit and PR title format
+
+We use [Conventional Commits](https://www.conventionalcommits.org/).
+
+Use:
+- `type(scope): description`
+- `type(scope)!: description` for breaking changes
+
+Allowed `type` values:
+- `feat`
+- `fix`
+- `chore`
+- `docs`
+- `test`
+- `refactor`
+- `perf`
+- `build`
+- `ci`
+- `revert`
+
+Examples:
+- `feat(nssa): add private PDA support`
+- `fix(wallet): correct fee calculation`
+- `feat(nssa)!: rename AccountId::from((prog, seed)) to AccountId::for_public_pda`
+
+Breaking changes:
+- Mark with `!` in the title.
+- Optionally add a `BREAKING CHANGE:` footer in the PR body with migration notes.
+
+`CHANGELOG.md` is generated from these markers on every `v*` tag via `git-cliff`, and GitHub Releases are created from the same content.
+
+Before merging PR consider squashing non-meaningful commits. E.g.:
+
+```
+- refactor(wallet): move user keys to a separate module
+- revert(wallet): revert "refactor(wallet): move user keys to a separate module"
+```
+
+Could be squashed to an empty commit if they belong to the same PR.
+
+## Branch workflow
+
+When bringing your feature branch up to date, prefer rebasing on top of `main`.
+
+- Preferred: `git rebase main`
+- Avoid: `git merge main` in feature branches
+
+This keeps commit history cleaner and makes reviews easier.
+
+## Useful commands
+
+We have [`Justfile`](./Justfile) which contains some useful utilities which may help you.
+
+To list all of them run the command: `just`.
+
+Any change to our core crates may invalidate our RISC0 [`artifacts`](./artifacts/), in that case you're required to run `just build-artifacts` to update them.
+
+## AI-assisted contributions
+
+AI tools are allowed for drafting code, docs, tests, and review suggestions.
+
+Requirements:
+- A human author is fully responsible for all submitted code and text.
+- The person opening the PR must review, verify, and be able to explain every change.
+- Do not open PRs automatically via AI agents or bots. Automatic AI-created PRs are not allowed.