docs(readme): replace V1 (IM client) with V2 (agentic company workspace)

This is the product's new V1 positioning for the 2026-05 launch.

Changes:
- Replace README.md with the Paperclip-structured V2 draft
  reviewed in joeylee12629-star/Joey-growth and live-previewed at
  nexu-readme-review.pages.dev
- Move the old README (IM-client narrative, MIT era) to
  docs/legacy-readme-v1-im-client.md for history

V2 structure (after 7 review rounds with Jinwei):
- Slack-bot hook intro
- What is Nexu (workspace where humans + agents run a company)
- 01/02/03 flow (channel = project)
- Works with 8 runtimes (Claude Code / OpenCode / Gemini CLI /
  OpenClaw / Pi / Cursor / Codex / Hermes)
- Nexu is right for you if (6 checkmarks)
- Features (6 cards)
- Use cases (Ship a feature / One-person company / Benchmark
  runtimes)
- Problems Nexu solves
- Nexu vs Slack (8-row comparison)
- What Nexu is not
- FAQ
- Share your ideas (X + GitHub issues)
- License (Apache 2.0) / Star History

Source of truth continues at
joeylee12629-star/Joey-growth (landing/readme-v2/nexu-readme-v2.md).

Note on license: README now shows Apache 2.0 per V2 positioning.
The LICENSE file in this repo is currently MIT -- that is a
separate follow-up (license-honesty thread) and is NOT changed by
this PR.

Author: joey