docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Setup
+
+```bash
+bun install
+bun run dev
+```
+
+Requires Bun `>=1.3.14` (see `package.json`'s `engines` field). See
+[README.md](README.md) for the dev loop, release process, and known
+Windows dev-mode issues.
+
+## Workflow
+
+Work here is issue-driven: every change starts from a GitHub issue, not
+from a conversation or a PR opened cold. See
+[AGENTS.md](AGENTS.md#workflow-follow-this-before-writing-any-code) for
+the full workflow this repo (and any AI agent working in it) follows.
+Short version:
+
+1. Pick an open issue, or file one if what you want to do isn't tracked
+   yet ([issue list](https://github.com/draylegend/letora/issues)).
+2. Create a branch from that issue's page on GitHub ("create a branch"
+   link) — this names it `<issue-number>-<slug>` for you.
+3. Check the branch out locally and implement.
+4. Open a PR against `dev` (the main branch here, not `main`).
+
+## Before opening a PR
+
+```bash
+bunx nx run-many -t lint test typecheck
+bunx nx build desktop --configuration=dev
+```
+
+This is what CI runs (`.github/workflows/ci.yml`). A `pre-commit` hook
+already runs formatting, lint --fix, and tests on staged files, and a
+`commit-msg` hook enforces commit message format — see below.
+
+## Commit messages
+
+Enforced by commitlint ([Conventional Commits](https://www.conventionalcommits.org/)):
+
+```
+<type>(<scope>): <short summary>
+```
+
+`type` is one of `feat`, `fix`, `chore`, `docs`, etc.; `scope` is
+optional and usually a project or area name (`desktop`, `eslint`,
+`deps`). Examples from this repo's history:
+
+```
+feat(desktop): add lint and typecheck coverage
+fix(eslint): configure tailwindcss plugin and error on violations
+chore(deps): update
+```