Fix annotation key mismatch: validate -> validation in generate.cr

anno[:validate] was always nil; validation procs were never wired
into Flag(T).new. Correct key is :validation to match the field
name set by the argument macro in command/argument.cr.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ATnpoEGXj4RyuMoBLGWcjs

src/cligen/app/generate.cr

@@ -27,7 +27,7 @@ module CliGen
               env_var: {% if anno[:env_var] %} {{anno[:env_var]}} {% else %} {{"#{cmd.name.split("::").last.upcase.id}_#{var.name.upcase}"}} {% end %},
               description: {{ anno[:description] }},
               default: {% unless var.default_value.nil? %} {{var.default_value}} {% else %} nil {% end %},
-              validate: {% if anno[:validate] %} {{anno[:validate]}} {% else %} nil {% end %},
+              validate: {% if anno[:validation] %} {{anno[:validation]}} {% else %} nil {% end %},
               on_match: {% if anno[:on_match] %} {{anno[:on_match]}} {% else %} nil {% end %}
             )
           {% debug if env("DEBUG") %}