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>

src/state-helper.ts

@@ -58,19 +58,3 @@ export function setDockerBuildStatus(dockerBuildStatus: string) {
 export function setDockerBuildDurationSeconds(dockerBuildDurationSeconds: string) {
   core.saveState('dockerBuildDurationSeconds', dockerBuildDurationSeconds);
 }
-
-export function setExposeId(exposeId: string) {
-  core.saveState('exposeId', exposeId);
-}
-
-export function getExposeId(): string {
-  return core.getState('exposeId');
-}
-
-export function setSetupOnly(setupOnly: boolean) {
-  core.saveState('setupOnly', setupOnly.toString());
-}
-
-export function getSetupOnly(): boolean {
-  return core.getState('setupOnly') === 'true';
-}