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>

tests/unit/client/test_webdav.py

@@ -354,3 +354,63 @@ async def test_remove_tag_from_file_not_assigned(mocker):
 
     # Verify result (should succeed even with 404)
     assert result is True
+
+
+@pytest.mark.unit
+async def test_get_files_by_tag_detects_directories(mocker):
+    """get_files_by_tag must flag tagged folders via <d:resourcetype/>.
+
+    Tagged folders need ``is_directory=True`` so the tag-exclusion layer
+    (issue #710) can hide their descendants.
+    """
+    mock_http_client = AsyncMock()
+    client = WebDAVClient(mock_http_client, "testuser")
+
+    # Two-entry response: one regular file, one collection (folder).
+    xml_content = b"""<?xml version="1.0"?>
+    <d:multistatus xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns">
+        <d:response>
+            <d:href>/remote.php/dav/files/testuser/Secret.txt</d:href>
+            <d:propstat>
+                <d:prop>
+                    <oc:fileid>101</oc:fileid>
+                    <d:displayname>Secret.txt</d:displayname>
+                    <d:getcontentlength>42</d:getcontentlength>
+                    <d:getcontenttype>text/plain</d:getcontenttype>
+                    <d:getlastmodified>Wed, 01 Jan 2025 00:00:00 GMT</d:getlastmodified>
+                    <d:getetag>"abc"</d:getetag>
+                    <d:resourcetype/>
+                </d:prop>
+            </d:propstat>
+        </d:response>
+        <d:response>
+            <d:href>/remote.php/dav/files/testuser/Private/</d:href>
+            <d:propstat>
+                <d:prop>
+                    <oc:fileid>102</oc:fileid>
+                    <d:displayname>Private</d:displayname>
+                    <d:getlastmodified>Wed, 01 Jan 2025 00:00:00 GMT</d:getlastmodified>
+                    <d:getetag>"def"</d:getetag>
+                    <d:resourcetype><d:collection/></d:resourcetype>
+                </d:prop>
+            </d:propstat>
+        </d:response>
+    </d:multistatus>"""
+
+    mock_response = AsyncMock()
+    mock_response.content = xml_content
+    mock_response.raise_for_status = mocker.Mock()
+    mock_http_client.request = AsyncMock(return_value=mock_response)
+
+    files = await client.get_files_by_tag(42)
+
+    assert len(files) == 2
+    by_path = {f["path"]: f for f in files}
+
+    assert by_path["/Secret.txt"]["is_directory"] is False
+    assert by_path["/Private/"]["is_directory"] is True
+
+    # Sanity-check the REPORT body asks for resourcetype.
+    call_args = mock_http_client.request.call_args
+    assert "<d:resourcetype/>" in call_args.kwargs["content"]
+    assert "<oc:systemtag>42</oc:systemtag>" in call_args.kwargs["content"]