docs: rewrite README around background, features, and the omp assistant

[danielglh]

What

Restructure the README so it reads like something a person wrote for people, in the order a newcomer actually needs:

1. Why this exists — the personal need up front: drive omp on a remote dev server from a browser, working well on both desktop and phone.
2. Core features — sessions/workspace rail, the chat UI (streaming, thinking blocks, tool cards, approvals), file browsing/previews, and first-class mobile drawers, each with a screenshot.
3. omp assistant — its own section making the design point explicit: one agent entry covers all omp configuration (via the real omp config CLI) plus omp-web's own session management, instead of building and maintaining a settings UI.
4. Install & use and Local development — trimmed to what's actionable; roadmap removed from the README.

Also adds a simplified-Chinese version at docs/README.zh-CN.md, cross-linked from both files.

Screenshots

All under docs/screenshots/ (7 PNGs, ~1.3 MB total). They were captured from a live instance running the bundled scripted agent with placeholder demo projects (/home/demo/website, /home/demo/api-server) in a throwaway container — nothing personal or real-project-related is visible in any of them.

Notes

• Pure markdown everywhere (the phone pair uses a table for side-by-side), so no sanitizer shows 'image blocked' placeholders.
• Roadmap items aren't lost — just no longer in the README; happy to move them to an issue if wanted.