fix: Add missing annotations for deck remove/unassign operations

- Add destructiveHint=True to deck_remove_label_from_card and
  deck_unassign_user_from_card (ADR-017 compliance)
- Set idempotentHint=True since remove operations produce same end state
- Update test_annotations.py to exclude nc_webdav_create_directory from
  non-idempotent check (MKCOL is idempotent by design - returns 405 if exists)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/server/deck.py

@@ -637,7 +637,9 @@ def configure_deck_tools(mcp: FastMCP):
 
     @mcp.tool(
         title="Remove Label from Deck Card",
-        annotations=ToolAnnotations(idempotentHint=False, openWorldHint=True),
+        annotations=ToolAnnotations(
+            destructiveHint=True, idempotentHint=True, openWorldHint=True
+        ),
     )
     @require_scopes("deck:write")
     @instrument_tool
@@ -692,7 +694,9 @@ def configure_deck_tools(mcp: FastMCP):
 
     @mcp.tool(
         title="Unassign User from Deck Card",
-        annotations=ToolAnnotations(idempotentHint=False, openWorldHint=True),
+        annotations=ToolAnnotations(
+            destructiveHint=True, idempotentHint=True, openWorldHint=True
+        ),
     )
     @require_scopes("deck:write")
     @instrument_tool