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 Delete the superseded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.

.agents/skills/gig-line-blender-asset-contract/references/asset-contract.md

@@ -1,7 +1,7 @@
 # Asset Contract Reference
 
 This is a working summary. The authoritative sources remain
-`docs/blender-assets.md`, `docs/asset-tooling.md`, ADR-0013, the tests in
+`docs/assets/blender-assets.md`, `docs/assets/asset-tooling.md`, ADR-0013, the tests in
 `src/test/tests/` and the constants in `tools/blender_assets.py` /
 `tools/npc_pipeline.py`.