docs: add agent-safe quick start and fix Seamless Auth naming

[Bccorb]

Part of fells-code/seamless-auth-api#383

Summary

• Adds a Quick start at the top of the README with the install line and the fully non-interactive scaffold command:
  seamless init my-app --local --yes --email=you@example.com
  It never prompts; with stdin from /dev/null it scaffolds the project, and a missing value exits 1 with an error naming the flag.
• First README sentence now describes the package as the CLI for Seamless Auth, open source passwordless auth. package.json description updated to match, and keywords added.
• Spells the product name as "Seamless Auth" in the init auth server prompt (and its tests and the README text that quotes it). Patch changeset included.
• Replaces the stale npx create-seamless reference in AGENTS.md with npx seamless-cli.

Checks

• npm run build (tsc) passes
• npm test: 61 files passed, 1245 tests passed
• npm run check:templates-pin and npm run check:types-current pass