feat(orchestrator): automatic provider fallback with runner availability check

Adds built-in load balancing: check GitHub runner availability before
builds start, auto-route to a fallback provider when runners are busy
or offline. Eliminates the need for a separate check-runner job.

New inputs: fallbackProviderStrategy, runnerCheckEnabled,
runnerCheckLabels, runnerCheckMinAvailable.

Outputs providerFallbackUsed and providerFallbackReason for workflow
visibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/model/build-parameters.ts

@@ -54,6 +54,10 @@ class BuildParameters {
   public sshAgent!: string;
   public sshPublicKeysDirectoryPath!: string;
   public providerStrategy!: string;
+  public fallbackProviderStrategy!: string;
+  public runnerCheckEnabled!: boolean;
+  public runnerCheckLabels!: string[];
+  public runnerCheckMinAvailable!: number;
   public gitPrivateToken!: string;
   public awsStackName!: string;
   public awsEndpoint?: string;
@@ -194,6 +198,10 @@ class BuildParameters {
       containerRegistryRepository: Input.containerRegistryRepository,
       containerRegistryImageVersion: Input.containerRegistryImageVersion,
       providerStrategy: OrchestratorOptions.providerStrategy,
+      fallbackProviderStrategy: OrchestratorOptions.fallbackProviderStrategy,
+      runnerCheckEnabled: OrchestratorOptions.runnerCheckEnabled,
+      runnerCheckLabels: OrchestratorOptions.runnerCheckLabels,
+      runnerCheckMinAvailable: OrchestratorOptions.runnerCheckMinAvailable,
       buildPlatform: OrchestratorOptions.buildPlatform,
       kubeConfig: OrchestratorOptions.kubeConfig,
       containerMemory: OrchestratorOptions.containerMemory,