docs(readme): full rewrite — sell the cockpit, ship 13 screenshots

The old README was a one-liner pointing at PLAN.md. The new one is a
proper landing page that:

- Opens with the cockpit hero shot + a tagline that explains why this
  template is different ("dev hub you'll actually want to use").
- Walks through every Dev Hub page (cockpit / features / tests /
  coverage / logs / diagnostics / JSON viewer / Scalar / admin tools)
  with a screenshot per surface — 13 captures at 1440x900 from
  http://localhost:3000 in headless Chromium.
- Tabulates all 14 toggleable features with default state + ENV key.
- Surfaces architecture, conventions, testing discipline, full tech
  stack, useful scripts, env-var cheatsheet, and links to PLAN.md /
  CLAUDE.md / docs/.
- Closes with a wink: "a dev hub that actually knallt".

Screenshots saved under docs/screenshots/. Project root now has a
proper landing for new contributors and template consumers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Ralph Loop