refactor: remove unused stickydisk setup logic and rename setup_builder

This commit completes the cleanup of build-push-action by removing all
the buildkit and sticky disk setup logic that has been moved to the
separate setup-docker-builder action.

Changes:
- Delete setup_builder.ts which contained 380+ lines of unused code
- Create new build-reporter.ts with only the reportBuildStart function
- Update all imports to use the new build-reporter module
- The new file name better reflects its single responsibility

The action is now cleaner and more focused, with infrastructure setup
logic properly separated into the setup-docker-builder action.

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

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

src/main.ts

@@ -18,7 +18,7 @@ import {ConfigFile} from '@docker/actions-toolkit/lib/types/docker/docker';
 
 import * as context from './context';
 import * as reporter from './reporter';
-import {reportBuildStart} from './setup_builder';
+import {reportBuildStart} from './build-reporter';
 import {Metric_MetricType} from '@buf/blacksmith_vm-agent.bufbuild_es/stickydisk/v1/stickydisk_pb';
 
 const DEFAULT_BUILDX_VERSION = 'v0.23.0';