docs: rewrite README to match the actual library

[joelshejar]

Summary

The previous README was a project-setup placeholder — "coming soon", empty checklist, "Like Istio for the frontend". It hasn't tracked reality since v1 landed (PR #4) and definitely doesn't reflect the 16 PRs that came after.

This rewrites it to ~200 lines covering what someone evaluating TabMesh actually needs to decide adoption.

Sections

• One-paragraph framing of the cross-tab pain it solves.
• Quickstart — minimal example + React snippet. Including `workerVersion` in the example so users land in the right deploy pattern from day one.
• Architecture — small diagram + brief mode summary.
• Gotchas — the section anyone evaluating should read first:
  • SharedWorker name caching → set `workerVersion` per deploy
  • `delivered` != backend-processed (will tighten with `ackMode: 'server'`)
  • No replay buffer for late-joining tabs (by design, per CONTEXT.md)
  • In-browser only, same-origin only
  • Mobile fallback paths get less coverage
  • SW handoff requires `deliveryUrl`
• Configuration — single-table reference cross-referencing `types.ts`.
• System events — the runtime-event surface for error handling.
• Development — the commands a contributor needs.
• Pre-1.0 status — explicit notice that the API may change.

Why prioritise gotchas

Anyone scanning a 200-line README skips ~80% of it. Putting the sharp edges right after Quickstart means the people who would have hit them and filed an issue self-select out before adopting — saving everyone time.

Test plan

• Renders cleanly on the GitHub markdown preview
• All linked paths exist (`CONTEXT.md`, `docs/adr/`, `packages/playground/scripts/build-worker.mjs`, `packages/core/src/types.ts`, `e2e/multi-tab.spec.ts`)