Add form-level validations to Tramway forms

[kalashnikovisme]

Motivation

• Provide form-level validation support analogous to normalizes so forms can validate inputs before saving.
• Ensure validations can be inherited and combined across form subclasses and integrated into the existing submit flow.

Description

• Add lib/tramway/forms/validations.rb implementing validates, validations, inheritance merging, and an __apply_validations method.
• Integrate validations into Tramway::BaseForm by requiring and including the module, initializing validations on inherited, applying validations during __submit, and making submit/submit! respect validation errors.
• Add example usage to README.md and guidance to docs/AGENTS.md about using form-level validates.
• Update dummy forms in spec/dummy/app/forms to declare validations and add spec/forms/validations_form_spec.rb to exercise validation behavior and inheritance.

Testing

• Added spec/forms/validations_form_spec.rb covering invalid attributes, allow_nil behavior, and validation inheritance.
• Attempted to run bundle exec rspec spec/forms/validations_form_spec.rb but the environment lacked bundler/rspec executables so the test run could not be executed here.

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Codex Task