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 Delete the superseded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-release-qa/references/qa-gates.md

@@ -93,7 +93,7 @@ python3 tools/screen_designs.py judge --model auto
 ```
 
 Record model, confidence/verdict summary, `diff` count and any manual review.
-`docs/ui-copy.md#screen-design-judge-report` is the durable record.
+`docs/design/ui-copy.md#screen-design-judge-report` is the durable record.
 
 ## 6. Realism judge (when visual assets/lighting changed)