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>

CHANGELOG.md

@@ -2,6 +2,20 @@
 
 All notable changes to this project will be documented in this file.
 
+## [0.9.1] - 2026-06-22
+
+### Fixed
+- `exec_in_container`: return a clear "not supported on this DSM firmware"
+  message instead of the raw DSM "Method does not exist" (code 103).
+- `redeploy_project`: corrected the failure hint that suggested
+  `stop_project + start_project` — `start_project` does not recreate on a new
+  image. It now points back to `redeploy_project` and warns about parallel
+  `pull_image` lock contention (DSM 2101/2202).
+
+### Changed
+- `check_image_updates`: docstring and output now state it only reflects digest
+  drift on already-pulled tags and does not discover newer version tags.
+
 ## [0.9.0] - 2026-06-16
 
 ### Changed (resilience against a slow/stalled NAS)