feat(template-runner): add Rails template post-action support

[dpaluy]

Summary

• Introduces new type: template post-action for executing full Rails application templates (including RailsBytes scripts)
• Adds TemplateRunner class wrapping Rails' AppGenerator with proper context injection and error handling
• Templates receive automatic @app_name and @answers instance variables with support for custom variables via config
• Includes TemplateError exception type for proper error wrapping and reporting
• Version bumped to 0.4.0 with comprehensive test coverage (39 test runs, 111 assertions)

Changes Included

• New file: lib/railstart/template_runner.rb (77 lines with YARD documentation)
• New test file: test/template_runner_test.rb (comprehensive coverage with mocks)
• New test file: test/cli_test.rb (CLI command tests)
• Enhanced lib/railstart/generator.rb with template execution flow and context handling
• Enhanced lib/railstart/config.rb with template action validation
• Updated lib/railstart/errors.rb with TemplateError exception class
• Updated lib/railstart/cli.rb with template command support
• Updated CHANGELOG.md and README.md with template documentation

Test plan

• Linting passes (bundle exec rubocop -A) - no offenses detected
• All existing tests pass (39 runs, 111 assertions)
• New TemplateRunner class tests cover:
  • Variable injection into template context
  • Destination root configuration
  • Error handling for missing source
  • Error wrapping for template execution failures
• Config validation for template post-actions verified
• Manual testing of template execution with variables