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/context.ts

@@ -58,7 +58,6 @@ export interface Inputs {
   ulimit: string[];
   'github-token': string;
   nofallback: boolean;
-  setupOnly: boolean;
   'buildx-version': string;
 }
 
@@ -98,7 +97,6 @@ export async function getInputs(): Promise<Inputs> {
     ulimit: Util.getInputList('ulimit', {ignoreComma: true}),
     'github-token': core.getInput('github-token'),
     nofallback: core.getBooleanInput('nofallback'),
-    setupOnly: core.getBooleanInput('setup-only'),
     'buildx-version': core.getInput('buildx-version')
   };
 }