docs: add contributing guide

CONTRIBUTING.md

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+# Contributing
+
+Thanks for helping improve Mike. Please keep contributions small, focused, and easy to review.
+
+## Guidelines
+
+- Prefer targeted edits over broad refactors.
+- Keep each PR focused on one bug, feature, or cleanup.
+- Update docs or env examples when changing setup, config, or user-facing behavior.
+- Please do not propose local-hosting refactors for the main app, such as local LLMs, local databases, or local filesystem storage. Those ideas are better suited to a future fully local version of the project.
+- Do not commit secrets, API keys, private documents, or local `.env` files.
+
+## Before Opening a PR
+
+- Run the relevant build or test command for the area you changed.
+- Check `git diff` and remove unrelated changes.
+- Write a concise Markdown PR description with:
+    - summary
+    - changes
+    - why
+    - testing
+
+## Local Development
+
+Backend:
+
+```bash
+npm run build --prefix backend
+```
+
+Frontend:
+
+```bash
+npm run build --prefix frontend
+```