chore: CI workflow + promote local skills feature in README

[scotthavird]

Summary

Two improvements that directly impact developer first impressions:

1. CI workflow — adds a green badge to the README and runs tests on every PR. Currently missing, which is a red flag for any developer evaluating the project.
2. README restructure — leads with the no-account value prop. Local skill generation is the fastest path to value for any developer with Claude Code installed, but it was buried after a 5-step platform quick start. Now it's the first thing you see.

Changes

CI

• `.github/workflows/test.yml` — runs `make test` + `make build` on push to main and all PRs
• CI badge added to README header

README

• New "No Account? Start Here" section at the top with a 2-command getting started flow
• Homebrew moved to top of installation options (most common path)
• Platform Quick Start section retitled and clearly scoped to account holders

Testing

• CI workflow syntax verified against existing release.yml patterns (Go 1.21, actions/checkout@v4, setup-go@v5)
• README links verified

Checklist

• Tests pass
• No breaking changes
• No platform internals