docs: rewrite README to reflect actual repository state

- Remove marketing fluff ("Revolutionary Feature") and personal project references
- Add accurate prerequisites section for new users
- Document the Tauri desktop app, diff viewer, commander, tabs, kanban board
- Update architecture section to reflect actual scale (83 server modules, 24 client modules, 116 tests)
- Add proper install instructions for both web and Windows desktop
- Document cascaded config system, CI workflows, dual-instance setup
- Replace stale internal PLANS/ references with user-facing content

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: web3dev1337