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/config.py

@@ -117,6 +117,10 @@ _DEFAULTS: dict[str, Any] = {
     "custom_processor_name": "custom",
     "custom_processor_api_key": None,
     "custom_processor_timeout": 60,
+    # Tag-based file exclusion (issue #710): comma-separated list of
+    # Nextcloud system tag names. Files/folders carrying any of these tags
+    # are hidden from WebDAV MCP tools. Empty = feature off.
+    "excluded_tags": "",
 }
 
 
@@ -508,6 +512,11 @@ class Settings:
     log_level: str = "INFO"
     log_include_trace_context: bool = True
 
+    # Tag-based file exclusion (issue #710): comma-separated list of
+    # Nextcloud system tag names. Files/folders carrying any of these tags
+    # are hidden from WebDAV MCP tools.
+    excluded_tags: str = ""
+
     def __post_init__(self):
         """Validate configuration and set defaults."""
         logger = logging.getLogger(__name__)
@@ -845,6 +854,7 @@ def get_settings() -> Settings:
         "log_format": "LOG_FORMAT",
         "log_level": "LOG_LEVEL",
         "log_include_trace_context": "LOG_INCLUDE_TRACE_CONTEXT",
+        "excluded_tags": "EXCLUDED_TAGS",
     }
 
     # Only pass values that dynaconf actually has; omit unset keys so