docs(ai): improve onboarding steps (agent)

- Fix Tailwind build warning by using pixel units for screens in `tailwind.config.cjs`.
- Update `CONTRIBUTING.md` to recommend sharded unit tests to avoid timeouts.
- Add `docs/onboarding-audit-20260128.md` with full audit report.

Co-authored-by: PR0M3TH3AN <[email protected]>

CONTRIBUTING.md

@@ -45,7 +45,7 @@ Automated agents contributing to this repository should follow these rules:
 
 1. Fork the repository and create a new branch from `main`.
 2. Make your changes, adding tests if applicable.
-3. Run `npm run format`, `npm run test:unit`, and `npm run lint` to ensure quality.
+3. Run `npm run format`, `npm run lint`, and unit tests (e.g. `npm run test:unit` or a shard like `npm run test:unit:shard1`) to ensure quality.
 4. (Optional) Run `npm run test:visual` if you made UI changes.
 5. (Optional) Run domain-specific tests if relevant (e.g., `npm run test:dm:unit` for Direct Messages).
 6. Push your branch and open a Pull Request against the `main` branch.
@@ -106,7 +106,7 @@ To set up the project locally:
    ```bash
    npm run test:unit
    ```
-   *Note: Unit tests may take a few minutes to complete. To speed up local iterations, you can run a specific shard (e.g., `npm run test:unit:shard1`).*
+   *Note: Running the full suite (`npm run test:unit`) is resource-intensive and may time out in some environments. We strongly recommend using sharded runs for local development: `npm run test:unit:shard1`.*
 
    You can also run end-to-end and visual tests:
    ```bash