refactor: complete removal of buildkit and sticky disk management

This commit completes the refactoring of build-push-action to focus solely on
Docker build reporting and metrics, with all infrastructure management moved
to the separate setup-docker-builder action.

Changes:
- Remove all setupOnly references from context.ts, main.ts, and state-helper.ts
- Rename startBlacksmithBuilder to reportBuildMetrics to better reflect its purpose
- Remove exposeId from all function signatures and state management
- Remove sticky disk commit logic from reporter.ts
- Update tests to match new function names and signatures
- Clean up unused imports and fix linting issues

The action now assumes that a Docker builder has already been configured
(either via setup-docker-builder or existing setup) and focuses only on:
- Running Docker builds with the configured builder
- Reporting build metrics and status to Blacksmith API
- Managing build outputs and metadata

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

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

babel.config.js

@@ -1,3 +1,4 @@
+// eslint-disable-next-line no-undef
 module.exports = {
   presets: [
     ['@babel/preset-env', { targets: { node: 'current' } }],