refactor: remove nofallback parameter

Remove the nofallback input parameter as it's no longer needed. The action
now assumes that a builder is already configured and available.

Changes:
- Remove nofallback from action.yml inputs
- Remove nofallback from Inputs interface and getInputs function
- Update tests to remove nofallback references
- Also remove setup-only and buildx-version from action.yml (already removed from code)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

action.yml

@@ -7,14 +7,6 @@ branding:
   color: 'blue'
 
 inputs:
-  nofallback:
-    description: "Fail the build if the remote builder is not available; defaults to false"
-    required: false
-    default: 'false'
-  setup-only:
-    description: "Only setup the Blacksmith builder without running a Docker build; defaults to false"
-    required: false
-    default: 'false'
   add-hosts:
     description: "List of a customs host-to-IP mapping (e.g., docker:10.180.0.1)"
     required: false
@@ -36,10 +28,6 @@ inputs:
   builder:
     description: "Builder instance"
     required: false
-  buildx-version:
-    description: "Version of Buildx to install"
-    required: false
-    default: 'v0.23.0'
   cache-from:
     description: "List of external cache sources for buildx (e.g., user/app:cache, type=local,src=path/to/dir)"
     required: false