Document the duckdb-ui exception to the separate-step rule

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -8,6 +8,7 @@
 - Do not use floating branches, unversioned archives, or implicit dependency downloads.
 - Add each dependency as a separate, reviewable build step so its source and version remain visible.
 - Append new dependency build steps after all existing dependency build steps. Do not insert or reorder steps in the existing build sequence, so previously successful build layers remain unchanged.
+- The DuckDB `ui` extension (`duckdb-ui`) is built in the same step as DuckDB instead of a separate step. It is statically linked into the DuckDB shell and can only be compiled as part of the DuckDB source tree (via `-DDUCKDB_EXTENSION_CONFIGS`), against the matching DuckDB version. Keep its pinned commit in the same `RUN` as DuckDB and update the two together. Because DuckDB is built from a tarball, pass `-DOVERRIDE_GIT_DESCRIBE` with the DuckDB version so the extension selects the correct API.
 - The log4cplus build is allowed to fetch its required `ThreadPool` submodule as a shallow repository pinned by `REL_2_0_5` to commit `cc0b6371d3963f7028c2da5fc007733f9f3bf205`. Do not fetch unused log4cplus submodules.
 
 ## System Development Packages