docs: rewrite README, add CONTRIBUTING, and polish project metadata

[awdr74100]

Summary

Polishes the project's open-source front door. No runtime/behavior changes beyond a pnpm-version requirement bump.

• README — full rewrite (the old one still read "M0 / Not yet usable"). Adds a Mermaid architecture diagram, Quick start, Skills, an 88-tool overview, an FAQ (covering the common npx + version-manager PATH gotcha with fnm/nvm/asdf/volta/mise), and a "What's in the name" note (the -wright maker tradition, a nod to Playwright).
• CONTRIBUTING.md — new contributor guide; points to AGENTS.md for the architecture deep-dive.
• npm metadata — sharpened the @figwright/mcp description and added keywords for discoverability; aligned the root description to match.
• pnpm 11 — bumped engines.pnpm to >=11.0.0 and aligned the docs (the project already pins pnpm@11.8.0 via packageManager).

Validation

• Local CI gate green: typecheck, lint, format:check, knip, build, test (664 tests).
• publint reports no issues for @figwright/mcp.

Out of band

• GitHub repo About + topics were updated separately and are already live (not part of this diff).