Copilot instructions for T3

# GitHub Copilot instructions

These instructions apply to the whole T3 repository. Copilot reads them from `.github/copilot-instructions.md`.
Prefer the commands and layout below over guesses.

## Stack

This repository is a T3 app: Next.js, tRPC, and the database client the repo already uses. Inputs are validated with Zod. The database is never imported into a client component.

## Layout

- Routers live in the server API tree the project already uses.
- The database client is imported only from server code.
- Shared Zod schemas live next to the procedure or in a shared validators module.

## Commands

- Dev server: `npm run dev`
- Typecheck: `npm run typecheck`
- Tests: `npm test`

Use the script the repo already defines when it differs from the command above.

## Conventions

- Every mutation input is a Zod schema. Do not accept `unknown` and cast it.
- Use the public procedure or protected procedure the router already exports.
- Do not import the database into a file marked `use client`.
- Environment variables go through the project's env schema.
- Return typed data the client can infer. Do not throw plain strings.
- Match the existing app-router or pages-router entry. Do not add the other one.

## Testing

Add or update tests when behavior changes.

Call the procedure with a caller or the project's test helper. Cover one invalid Zod input.

## Style

- Optimize for the next reader over the shortest diff.
- Comment only when the why is non-obvious.
- Name things the way neighboring files already do.
- Do not reformat code you did not change.

## Errors

- Fail at the boundary. Do not swallow errors.
- Use TRPCError with a code for expected failures. Unexpected errors can propagate.

## Scope

- Do the requested task. Do not refactor unrelated code.
- Match the rigor already in the repo. Fix what you touch.
- Ask one clarifying question only when the task cannot be finished without it. Otherwise state the assumption and continue.

## Avoid

- Do not call the database from a React component to 'save a round trip'.
- Do not skip the protected procedure and read the session by hand in a new style.
- Do not add REST routes next to tRPC for the same resource.
- Do not commit `.env`. The env schema should fail fast when a variable is missing.

## Done when

- The requested behavior works.
- `npm run typecheck` passes and the procedure rejects an invalid input.
- The diff stays inside the task.
- You list anything you could not run.

## Review checklist

- The change matches the commands and layout in this file.
- No new dependency was added without a reason already present in the repo.
- Protected procedures check the session. A hidden button is not auth.
- Zod-parse every input, including ids.
- Keep database URLs and auth secrets in server env vars.