Add more thorough README

# Why?

**As a** developer setting up this repo on a new machine
**I want** the README to explain what this service is and how to get a working local setup
**So that** I'm not reverse-engineering `package.json` scripts, `amplify.yml`, and CLAUDE.md to figure out prerequisites, env vars, and commands from scratch.

# What?

The current README is a two-line stub (`# rms-app` / `Reservation Management System App`). Replace it with:

1. **What this service is**: a short description of RMS (reservation/borrow-tracking system for items/inventory), its two main pieces (`ts-code/` shared backend logic on AWS Amplify Gen 2 — Cognito/Lambda/DynamoDB — and the `web/` Next.js frontend), and a pointer to `CLAUDE.md` for full architectural detail rather than duplicating it.

2. **New-machine setup instructions**, covering:
- Prerequisites: Node version, `npm install` at repo root and in `web/`.
- AWS CLI setup: the `rms-alpha` profile (`AWS_PROFILE=rms-alpha`), where credentials live (`~/.aws/credentials`), and `aws sts get-caller-identity` to verify.
- `gh` CLI auth (`gh auth login`, `gh auth status`).
- Backend build/test commands (`npm run build`, `npm run build:gen2`, `npm run test:unit`, `npm run test:integ`).
- `web/` local dev setup: `.env.local` from `.env.local.example`, `npm run dev`, `npm run test`, `npm run test:e2e` (and the `RMS_TEST_USER_EMAIL`/`RMS_TEST_USER_PASSWORD`/`RMS_TEST_ITEM_ID` env vars Playwright specs need).
- Branching convention pointer: `alpha` is the default/PR-target branch, not `master` (link to CLAUDE.md's "Branching and environment promotion" section rather than re-explaining it).

# Testing

Documentation-only change — no test required.

# Additional context

Requested directly by the repo owner, who is setting this repo up on a new computer and wants this documented for that purpose.