feat: add Nextcloud Collectives app support (#621)

Implement MCP tools for the Collectives wiki/documentation app, enabling
agentic workflows for team knowledge base management.

16 tools covering collectives, pages, tags, search, and trash:
- Read: list collectives, list/get pages (with WebDAV content), search,
  list tags, list trashed pages
- Write: create/update collective, create/move/trash/restore pages,
  set emoji, create/assign/remove tags

Includes Docker hook for app installation, OCS API client with envelope
unwrapping, Pydantic models, unit tests (16), and integration tests (10).

Closes #621

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/__init__.py

@@ -14,6 +14,7 @@ from httpx import (
 from ..controllers.notes_search import NotesSearchController
 from ..http import nextcloud_httpx_transport
 from .calendar import CalendarClient
+from .collectives import CollectivesClient
 from .contacts import ContactsClient
 from .cookbook import CookbookClient
 from .deck import DeckClient
@@ -81,6 +82,7 @@ class NextcloudClient:
         )  # Uses AsyncDavClient internally
         self.contacts = ContactsClient(self._client, username)
         self.cookbook = CookbookClient(self._client, username)
+        self.collectives = CollectivesClient(self._client, username)
         self.deck = DeckClient(self._client, username)
         self.news = NewsClient(self._client, username)
         self.users = UsersClient(self._client, username)