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/models/auth.py

@@ -74,5 +74,7 @@ ALL_SUPPORTED_SCOPES: frozenset[str] = frozenset(
         "sharing:write",
         "news:read",
         "news:write",
+        "collectives:read",
+        "collectives:write",
     }
 )