Installer site/contrib config seeding

[UltraBob]

Implement the installer's site/contrib question and config seeding. The "mode" lives in what gets seeded, not in a runtime variable.

Work items

• Installer asks site or contrib at install time
• Site: seed configs at project root with custom-path directives
• Contrib: don't seed by default — rely on the cascade
• Provide explicit seed command/option for contrib maintainers who want to pin settings
• Drop runtime curl from phpcs command
• Confirm seeded configs carry no #ddev-generated marker

Acceptance criteria

• Installer prompts for site or contrib and seeds accordingly
• Site seeding writes configs with custom-path directives
• Contrib install does not seed by default
• Explicit seed command available for contrib opt-in
• Runtime curl removed from phpcs
• Seeded configs carry no #ddev-generated marker

Blocked by: ADRs for position-aware defaults and config provenance (PRs pending)