feat(channel): deliver generated files on WhatsApp and honor user tool policy

Add save_deliverable_attachment for explicit document outbound, pass channel user_id into tool risk gating so write_file/run_command respect per-user allow_high settings, and document the workflow in channel-file-delivery skill.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_save_deliverable_attachment_tool.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from runtime.tools.public.save_deliverable_attachment_tool import save_deliverable_attachment_tool
+
+
+def test_save_deliverable_attachment_registers_file(tmp_path, monkeypatch) -> None:
+    from svc.files.attachment_assets import AttachmentAssetStore
+
+    store = AttachmentAssetStore(root_dir=tmp_path / "att")
+    monkeypatch.setattr(
+        "runtime.tools.public.save_deliverable_attachment_tool.AttachmentAssetStore",
+        lambda root_dir=None: store if root_dir is None else AttachmentAssetStore(root_dir=root_dir),
+    )
+    monkeypatch.setattr(
+        "runtime.tools.public.save_deliverable_attachment_tool.resolve_workspace_path",
+        lambda raw: tmp_path / "report.txt",
+    )
+    report = tmp_path / "report.txt"
+    report.write_text("hello deliverable\n", encoding="utf-8")
+
+    spec = save_deliverable_attachment_tool()
+    out = spec.handler({"path": "report.txt"})
+    assert out.get("ok") is True
+    assert out.get("deliverable") is True
+    assert out.get("attachment_id")
+    assert out.get("mime") == "text/plain"
+    blob, meta = store.load_bytes(str(out["attachment_id"]))
+    assert blob is not None
+    assert blob.decode("utf-8").replace("\r\n", "\n") == "hello deliverable\n"
+    assert meta is not None