Reorganize docs into category folders and drop the retired head-review page

Group the top-level docs into docs/project, docs/design, docs/assets and
docs/process, each with its own README index. Delete the superseded
docs/screens/head-review page and sheets. Rewrite every markdown link,
absolute docs path and tool/test reference to the new locations, fix the
pre-existing broken anchors and remove the dead Joke Shop photo-board link.
check_docs OK, Python tooling 167/167.

tools/README.md

@@ -160,7 +160,7 @@ dependencies or replace the tools above.
   network/budget-dependent review aids, not deterministic CI gates.
 - `openrouter_judge.py` is an implementation detail imported by other tools.
 - `check_docs.py` and `export_venues.gd` are short, single-purpose commands;
-  documenting them in this README and `docs/asset-tooling.md` is enough.
+  documenting them in this README and `docs/assets/asset-tooling.md` is enough.
 - `npc_baseline.py`, `npc_proportions.py` and `three_up.py` are better included
   as references/steps inside the proposed asset or release skill than exposed as
   separate skills; fragmenting them would increase routing mistakes.