feat: add compute gateway step

Author: intel352

README.md

@@ -83,6 +83,38 @@ Checks or polls the status of a GitHub Actions workflow run.
     timeout: "30m"
 ```
 
+### Step: `step.gh_compute_gateway`
+
+Submits a GitHub-origin workload to workflow-compute's protected gateway. The
+compute server remains in the execution path; raw agents are not registered as
+GitHub runners. When `write_check` is enabled, the check target must match the
+repository and SHA verified by the compute server.
+
+```yaml
+- type: step.gh_compute_gateway
+  config:
+    server_url: "${COMPUTE_SERVER_URL}"
+    token: "${COMPUTE_GITHUB_TOKEN}"
+    repository: "GoCodeAlone/workflow-compute"
+    oidc_token: "{{.github_oidc_token}}"
+    workflow_run_id: "{{.run_id}}"
+    workflow_job_id: "{{.job_id}}"
+    workflow_job_name: "build"
+    ref: "{{.ref}}"
+    sha: "{{.sha}}"
+    org_id: "org-1"
+    pool_id: "pool-1"
+    policy_id: "policy-1"
+    command_args: ["go", "test", "./..."]
+    wait: true
+    write_check: true
+    check_owner: "GoCodeAlone"
+    check_repo: "workflow-compute"
+    check_sha: "{{.sha}}"
+    check_name: "workflow-compute"
+    check_token: "${GITHUB_TOKEN}"
+```
+
 ### Step: `step.gh_create_check`
 
 Creates a GitHub Check Run (commit status check).

internal/plugin.go

@@ -71,6 +71,7 @@ func (p *githubPlugin) StepTypes() []string {
 		// Existing steps
 		"step.gh_action_trigger",
 		"step.gh_action_status",
+		"step.gh_compute_gateway",
 		"step.gh_create_check",
 		// Pull request steps
 		"step.gh_pr_create",
@@ -100,6 +101,8 @@ func (p *githubPlugin) CreateStep(typeName, name string, config map[string]any)
 		return newActionTriggerStep(name, config, nil)
 	case "step.gh_action_status":
 		return newActionStatusStep(name, config, nil)
+	case "step.gh_compute_gateway":
+		return newComputeGatewayStep(name, config, nil)
 	case "step.gh_create_check":
 		return newCreateCheckStep(name, config, nil)
 	case "step.gh_pr_create":