fix(collaboration): harden delegated task isolation (#1486)

* feat(collaboration): isolate delegated artifacts and child runs

Isolate delegated inputs and outputs behind secure artifact exchange lifecycles. Scope Agent Link tasks by tenant and root agent, and enforce delegation spawn-tree boundaries. Add process-wide child-run admission and preserve logical media paths across native, MCP, and sandbox execution.

* fix(collaboration): harden delegated task isolation

Enforce tenant and root-agent task scope across migrations and stores. Add exactly-once async completion delivery, delegated sandbox boundaries, and confined artifact and media recovery across runtime surfaces.

* fix(collaboration): recover interrupted async tasks

* fix(collaboration): normalize persisted child-run status

---------

Co-authored-by: ntduc <ntduc@cpp.ai.vn>

internal/tools/delegation_artifacts_linux.go

@@ -0,0 +1,32 @@
+//go:build linux
+
+package tools
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+func (r *artifactSecureRoot) renameNoReplace(oldName, newName string) error {
+	oldPath, err := validateArtifactRelativePath(oldName)
+	if err != nil {
+		return err
+	}
+	newPath, err := validateArtifactRelativePath(newName)
+	if err != nil {
+		return err
+	}
+	err = unix.Renameat2(r.fd, oldPath, r.fd, newPath, unix.RENAME_NOREPLACE)
+	if errors.Is(err, unix.EEXIST) {
+		return errors.Join(ErrArtifactPublishConflict, err)
+	}
+	if errors.Is(err, unix.ENOSYS) || errors.Is(err, unix.EINVAL) {
+		return errors.Join(ErrArtifactSecureUnavailable, err)
+	}
+	return err
+}
+
+func isArtifactNotExist(err error) bool {
+	return errors.Is(err, unix.ENOENT)
+}