feat(webdav): add tag-based file exclusion (#710)

Hide sensitive files/folders from the WebDAV MCP tool surface by
tagging them with a configured Nextcloud system tag. Defence-in-depth
control for users who connect LLMs to accounts holding contracts,
medical records, credentials, etc.

A new EXCLUDED_TAGS env var (comma-separated tag names, empty by
default) gates an exclusion layer that runs at the start of every
WebDAV tool call: tag names are resolved to tag IDs, those IDs are
expanded to the set of tagged paths, then listings/searches are
filtered and read/write/delete/move/copy operations on excluded paths
raise ToolError. Tagged folders exclude their descendants via prefix
match. Empty EXCLUDED_TAGS disables the feature entirely.

The threat model is preventing accidental data exfiltration via the
LLM tool surface — not hiding files from a determined operator. The
docs explicitly recommend creating exclusion tags with
user_assignable=false so the credentials the MCP server uses cannot
remove the tag.

Implementation:

- config.py: add `excluded_tags` to _DEFAULTS, Settings, and the
  _field_map alongside other comma-separated env vars.
- client/webdav.py: get_files_by_tag now requests <d:resourcetype/>
  and surfaces is_directory so tagged directories can recursively
  exclude descendants.
- server/tag_exclusion.py (new): get_excluded_tag_names,
  get_excluded_file_paths, is_path_excluded.
- server/webdav.py: exclusion guards in all 11 WebDAV tools;
  read/write/create/delete/move/copy raise ToolError, list/search
  tools silently filter excluded entries. Existing f-string log
  calls converted to lazy %-style.
- tests: 17 new unit tests covering path-matching edge cases
  (shared-prefix non-match, descendants of excluded dirs), tag-name
  parsing, and get_excluded_file_paths with mocked WebDAV; 1 new
  client test asserting <d:resourcetype/> -> is_directory parsing.
- docs/configuration.md: new "Tag-Based File Exclusion" section with
  per-tool effect table, security guidance, and per-call cost note.
- README.md: feature mention under Key Features.

Closes #710.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/webdav.py

@@ -1197,7 +1197,9 @@ class WebDAVClient(BaseNextcloudClient):
         Returns:
             List of file info dictionaries with path, size, content_type, etc.
         """
-        # Use WebDAV REPORT method with systemtag filter, requesting all properties
+        # Use WebDAV REPORT method with systemtag filter. resourcetype is
+        # included so callers can distinguish folders from files (needed for
+        # recursive exclusion of tagged directories — see issue #710).
         report_body = f"""<?xml version="1.0"?>
 <oc:filter-files xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns">
   <d:prop>
@@ -1207,6 +1209,7 @@ class WebDAVClient(BaseNextcloudClient):
     <d:getcontenttype/>
     <d:getlastmodified/>
     <d:getetag/>
+    <d:resourcetype/>
   </d:prop>
   <oc:filter-rules>
     <oc:systemtag>{tag_id}</oc:systemtag>
@@ -1249,10 +1252,17 @@ class WebDAVClient(BaseNextcloudClient):
             contenttype_elem = prop.find("d:getcontenttype", ns)
             lastmodified_elem = prop.find("d:getlastmodified", ns)
             etag_elem = prop.find("d:getetag", ns)
+            resourcetype_elem = prop.find("d:resourcetype", ns)
 
             if fileid_elem is None or not fileid_elem.text:
                 continue
 
+            # A resourcetype with a <d:collection/> child indicates a folder.
+            is_directory = (
+                resourcetype_elem is not None
+                and resourcetype_elem.find("d:collection", ns) is not None
+            )
+
             # Decode href path and extract the file path
             href_path = unquote(href_elem.text)
             # Remove WebDAV prefix to get user-relative path
@@ -1285,10 +1295,11 @@ class WebDAVClient(BaseNextcloudClient):
                 else None,
                 "last_modified_timestamp": last_modified_timestamp,
                 "etag": etag_elem.text if etag_elem is not None else None,
+                "is_directory": is_directory,
             }
             files.append(file_info)
 
-        logger.debug(f"Found {len(files)} files with tag ID {tag_id}")
+        logger.debug("Found %d files with tag ID %s", len(files), tag_id)
         return files
 
     async def get_file_info(self, path: str) -> dict[str, Any] | None: