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 Delete the superseded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/verification.md

@@ -113,8 +113,8 @@ For each changed variant, inspect at least:
 - Day and night/LATE final-context frames.
 - Front, three-quarter and profile for a shape change.
 
-Accepted stylized limits are in `docs/3d-model-review.md` and
-`docs/known-limitations.md`. Do not turn an accepted limit into an unreviewed
+Accepted stylized limits are in `docs/assets/3d-model-review.md` and
+`docs/design/known-limitations.md`. Do not turn an accepted limit into an unreviewed
 contract change.
 
 ## Final diff review