Allow explicit feature creation in strict mode

[oiahoon]

Fixes #1026

Summary

• let explicit Flipper.add calls use the strict adapter's existing scoped check bypass
• keep strict checks active for implicit feature creation through operations such as enable
• add regression coverage for both behaviors

Test plan

• bundle exec rspec spec/flipper/dsl_spec.rb spec/flipper/adapters/strict_spec.rb
• core RSpec suite excluding service/socket/terminal-dependent files (2,595 examples, 0 failures)
• core Minitest suite excluding Redis-dependent files (242 runs, 4,636 assertions, 0 failures)
• Rails generator and helper tests (14 runs, 111 assertions, 0 failures)
• bundle exec rake build