Add diagnostic logging to delete_image for DSM error 114

- Log api, method, name, tag to stderr immediately before the DSM call
- Log the DSM error code on failure (visible in Claude Desktop stderr)
- Include DSM error code in the return message so it appears in chat

This makes the exact request params and the DSM code visible without
needing debug-level logging enabled, to help diagnose why error 114
("Invalid API call") is returned by SYNO.Docker.Image/delete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mcp_synology_container/modules/images.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import logging
+import sys
 from datetime import UTC, datetime
 from typing import TYPE_CHECKING, Any
 
@@ -242,9 +243,15 @@ def register_images(mcp: FastMCP, config: AppConfig, client: DsmClient) -> None:
                 f"Call delete_image(image_id={image_id!r}, confirmed=True) to confirm."
             )
 
-        # Perform deletion using the resolved name+tag
+        # Perform deletion using the resolved name+tag.
+        # N4S4 reference: SYNO.Docker.Image / delete, params: name, tag
         delete_name = repo
         delete_tag = img_tags[0] if img_tags else tag
+        sys.stderr.write(
+            f"[delete_image] api=SYNO.Docker.Image method=delete"
+            f" name={delete_name!r} tag={delete_tag!r}\n"
+        )
+        sys.stderr.flush()
         try:
             await client.request(
                 "SYNO.Docker.Image",
@@ -252,7 +259,10 @@ def register_images(mcp: FastMCP, config: AppConfig, client: DsmClient) -> None:
                 params={"name": delete_name, "tag": delete_tag},
             )
         except Exception as e:
-            return f"Error deleting image '{display_name}': {e}"
+            code = getattr(e, "code", "?")
+            sys.stderr.write(f"[delete_image] failed: {e} (DSM code {code})\n")
+            sys.stderr.flush()
+            return f"Error deleting '{display_name}': {e} [DSM code {code}]"
 
         return f"Deleted {display_name} — {size_str} freed."