docs: improve README onboarding and reference

[mastertyko]

Summary

• Lead with a three-step install and readiness path.
• Separate native Computer Use from the one-owned-tab Chrome surface.
• Replace long prose lists with scannable tool, command, requirement,
  configuration, and troubleshooting tables.
• Clarify permissions, fail-closed behavior, trust probing, uncertain outcomes,
  development checks, releases, and project links.

Accuracy checks

• All 12 Computer Use tools and all 3 Chrome tools match source registration.
• All 8 /codex-computer commands match the command handler.
• All 6 documented environment variables match runtime source.
• Unsupported providers, existing tabs, raw JavaScript, and CDP remain
  explicitly out of scope.
• Poisoned Chrome runs are not described as guaranteed tab cleanup.

Verification

• markdownlint: 0 issues with line-length disabled for tables and URLs
• bun run check: 21 files, 233 tests
• npm pack --dry-run
• Gitleaks: no leaks
• GitHub and badge links: HTTP 200
• npm page: verified through Chrome