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/server/__init__.py

@@ -1,4 +1,5 @@
 from .calendar import configure_calendar_tools
+from .collectives import configure_collectives_tools
 from .contacts import configure_contacts_tools
 from .cookbook import configure_cookbook_tools
 from .deck import configure_deck_tools
@@ -11,6 +12,7 @@ from .webdav import configure_webdav_tools
 
 __all__ = [
     "configure_calendar_tools",
+    "configure_collectives_tools",
     "configure_contacts_tools",
     "configure_cookbook_tools",
     "configure_deck_tools",