fix: v0.9.1 — clearer messages for exec/redeploy/check_image_updates

Three live-verified MCP weaknesses (update run 2026-06-22):

- exec_in_container: SYNO.Docker.Container/exec does not exist on this DSM
  firmware (code 103, like pause/unpause). Catch it and return a guiding
  "not supported on this firmware" message instead of the raw DSM error;
  the tool stays available for firmwares that do implement exec.
- redeploy_project: the failure hint suggested stop_project + start_project,
  but start_project does not recreate on a new image (it restarts the old
  one). Both failure hints now point back at redeploy_project and warn about
  parallel pull_image lock contention (DSM 2101/2202).
- check_image_updates: docstring and output now state it only reflects DSM
  digest drift on already-pulled tags and does NOT discover newer version
  tags (use list_image_tags + release notes for that).

Tests updated accordingly; CHANGELOG + version bumped to 0.9.1.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_modules/test_images.py

@@ -720,6 +720,7 @@ async def test_check_image_updates_all():
     assert "nginx:1.24" in result
     assert "UPDATE AVAILABLE" in result
     assert "postgres:15" in result
+    assert "does NOT detect newer version tags" in result
 
 
 @pytest.mark.asyncio
@@ -744,6 +745,7 @@ async def test_check_image_updates_all_up_to_date():
 
     result = await tools["check_image_updates"]()
     assert "All images are up to date" in result
+    assert "does NOT detect newer version tags" in result
 
 
 @pytest.mark.asyncio