Add signup JWT to store auth

[dengjeffrey]

Summary

• Adds a --signup flag to shopify store auth.
• Plumbs the signup JWT through store auth into the PKCE bootstrap.
• Appends signup=<JWT> to the Admin OAuth /admin/oauth/authorize URL so Core/Admin can process the Projects.dev store signup before scope approval.

Tests

• pnpm vitest run packages/store/src/cli/commands/store/auth.test.ts packages/store/src/cli/services/store/auth/pkce.test.ts packages/store/src/cli/services/store/auth/index.test.ts
• pnpm nx run store:type-check

Notes

• This is a POC companion to the Core Projects.dev signup JWT handling branch.
• The JWT is forwarded only to the OAuth authorize URL; it is not persisted by the CLI.