feat: deprecate shiny add test and disable test-gen prompt verification

[karangattu]

Uses click's built-in deprecated=True (red DeprecationWarning on stderr + (Deprecated) label in --help) together with a custom yellow warning, the way pip and Homebrew surface deprecations:

DeprecationWarning: The command 'test' is deprecated.
! `shiny add test` is deprecated and will be removed within the next few releases.
  Use the `shiny-for-python` skill with an agent harness to generate tests instead.

Also disables the verify-test-generation-prompts workflow (manual dispatch only).