feat: bind Plugin Host identity to live Worker authority

crates/workspace-server/src/server/subject_authorization.rs

@@ -314,6 +314,37 @@ pub(super) fn worker_permissions(
     ))
 }
 
+/// Host-only identity shared by plan creation and live Plugin operations. The
+/// identity is an immutable ceiling, not a permission: recheck the live lineage
+/// on every use, including byte delivery and cache hits.
+pub(super) fn plugin_execution_identity(
+    api: &WorkspaceApi,
+    worker: &RuntimeWorkerRef,
+) -> ApiResult<crate::plugin_plan::HostExecutionIdentity> {
+    worker_permissions(api, worker)?;
+    let (principal_id, delegation_id) = if let Some(grant) = live_child_grant(api, worker)? {
+        (
+            format!("subject:{}", grant.subject_id),
+            Some(grant.source_change_id),
+        )
+    } else if let Some(subject) = current_body_subject(api, worker)? {
+        (format!("subject:{subject}"), None)
+    } else if let Some(subject) = job_subject(api, worker)? {
+        (format!("subject:{subject}"), None)
+    } else {
+        (
+            format!("worker:{}/{}", worker.runtime_id, worker.worker_id),
+            None,
+        )
+    };
+    Ok(crate::plugin_plan::HostExecutionIdentity {
+        runtime_id: worker.runtime_id.clone(),
+        worker_id: worker.worker_id.clone(),
+        principal_id,
+        delegation_id,
+    })
+}
+
 pub(super) fn authorize(
     api: &WorkspaceApi,
     worker: &RuntimeWorkerRef,