docs: add OpenAPI support requirement to spec, architecture, and plan

docs/ARCHITECTURE.md

@@ -141,6 +141,7 @@ The server is architected from the ground up to support concurrent, multi-user o
 | `openproject_list_statuses` | List all available work package statuses (New, In Progress, Closed, etc.) | None |
 | `openproject_list_priorities` | List all priority levels | None |
 | `openproject_list_users` | List users in the OpenProject instance | `pageSize`, `offset` |
+| `openproject_get_openapi_spec` | Retrieve OpenProject API v3 OpenAPI specification for schema introspection | `path` |
 
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