docs: add CONTRIBUTING.md guide for new contributors

CONTRIBUTING.md

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+# Contributing to CodeNomad
+
+Thank you for your interest in contributing! This guide will help you get started.
+
+## Quick Start
+
+```bash
+git clone https://github.com/NeuralNomadsAI/CodeNomad.git
+cd CodeNomad
+npm install
+npm run dev
+```
+
+## Finding Issues to Work On
+
+Browse [open issues](https://github.com/NeuralNomadsAI/CodeNomad/issues) and look for these labels:
+
+| Label | Meaning |
+|---|---|
+| `ready-to-work` | Clear scope, ready for anyone to pick up |
+| `good-first-issue` | Good for first-time contributors |
+| `enhancement` | New feature requests |
+| `bug` | Bug reports |
+
+**Before starting:** comment on the issue so we can discuss approach and avoid duplicate work.
+
+## Development Workflow
+
+### 1. Fork and Branch
+
+```bash
+# Fork the repo on GitHub, then clone your fork
+git clone https://github.com/YOUR_USERNAME/CodeNomad.git
+cd CodeNomad
+
+# Add the upstream remote
+git remote add upstream https://github.com/NeuralNomadsAI/CodeNomad.git
+
+# Create a branch from upstream/dev
+git fetch upstream
+git checkout -b fix/your-branch-name upstream/dev
+```
+
+### 2. Branch Naming
+
+| Prefix | Use for |
+|---|---|
+| `fix/` | Bug fixes |
+| `feat/` | New features |
+| `docs/` | Documentation changes |
+| `refactor/` | Code refactoring |
+| `chore/` | Build, config, maintenance |
+
+Examples: `fix/question-queue-ordering`, `feat/retry-tool-call`, `docs/contributing-guide`
+
+### 3. Make Your Changes
+
+```bash
+# Install dependencies
+npm install
+
+# Run the dev server
+npm run dev
+
+# Run type checking
+npm run typecheck --workspace @codenomad/ui
+```
+
+### 4. Commit
+
+Write clear, descriptive commit messages. Explain **what** changed and **why**.
+
+```bash
+git add .
+git commit -m "fix(ui): preserve question queue order when upserting duplicate requests
+
+When a question arrives as a global entry and later resolves to a tool
+part with a newer timestamp, the original enqueue time was lost, causing
+the question to move behind newer entries and break interruption order."
+```
+
+### 5. Push and Create a PR
+
+```bash
+git push origin your-branch-name
+```
+
+Then open a pull request on GitHub targeting the `dev` branch.
+
+**PR checklist:**
+- [ ] Branch is based on latest `upstream/dev`
+- [ ] One issue per PR (don't mix unrelated changes)
+- [ ] `npm run typecheck --workspace @codenomad/ui` passes
+- [ ] Tests pass (if applicable)
+- [ ] PR description explains the change and links the issue
+
+## Project Structure
+
+| Package | Description |
+|---|---|
+| `packages/server` | Core logic & CLI — workspaces, OpenCode proxy, API, auth |
+| `packages/ui` | SolidJS frontend — reactive UI components and stores |
+| `packages/electron-app` | Electron desktop shell |
+| `packages/tauri-app` | Tauri desktop shell (experimental) |
+
+### Key UI Files
+
+| Path | Purpose |
+|---|---|
+| `packages/ui/src/stores/session-events.ts` | SSE event handlers (idle, status, permissions, questions) |
+| `packages/ui/src/stores/session-actions.ts` | User actions (send message, abort, revert, fork) |
+| `packages/ui/src/stores/message-v2/` | Message store (v2 architecture) |
+| `packages/ui/src/stores/instances.ts` | Instance management and interruption queues |
+| `packages/ui/src/components/tool-call.tsx` | Tool call rendering |
+| `packages/ui/src/components/message-block.tsx` | Message display blocks |
+| `packages/ui/src/components/session/session-view.tsx` | Main session view |
+| `packages/ui/src/lib/i18n/messages/` | Translation files (en, es, fr, ja, ru, he, zh-Hans) |
+
+### Styling
+
+- Tokens: `src/styles/tokens.css`
+- Utilities: `src/styles/utilities.css`
+- Component styles: `src/styles/components/`, `src/styles/messaging/`, `src/styles/panels/`
+- Keep style files under ~150 lines; split by component
+
+### Internationalization (i18n)
+
+- Use `useI18n()` in components, `tGlobal()` in stores
+- Messages live in `packages/ui/src/lib/i18n/messages/<locale>/`
+- When adding a string: add to `en/` first, then add the same key to every other locale
+- Placeholders use `{name}` syntax (word characters only)
+
+## Code Principles
+
+- **KISS**: Keep modules narrowly scoped
+- **DRY**: Share helpers before copy-pasting
+- **Single responsibility**: Split files when concerns diverge
+- **Composable primitives**: Prefer signals, hooks, utilities over deep inheritance
+
+## Need Help?
+
+- Check existing [issues](https://github.com/NeuralNomadsAI/CodeNomad/issues) and [PRs](https://github.com/NeuralNomadsAI/CodeNomad/pulls)
+- Ask in the issue you're working on
+- Review the [server documentation](packages/server/README.md) for CLI flags and configuration