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 Delete the superseded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.

docs/design/README.md

@@ -0,0 +1,13 @@
+# Design Documentation
+
+Gameplay, venues, presentation and player-facing constraints.
+
+- [`gameplay.md`](gameplay.md) — the playable loop, escalation mechanics, venues and tutorial.
+- [`venues.md`](venues.md) — real-site evidence, game mapping and venue simplifications.
+- [`ui-copy.md`](ui-copy.md) — screen, copy and button inventory plus the screen-design judge
+  record.
+- [`accessibility.md`](accessibility.md) — accessibility statement and known gaps.
+- [`known-limitations.md`](known-limitations.md) — release blockers and accepted limitations.
+
+Up: [`../README.md`](../README.md) · Product plan: [`../../PLAN.md`](../../PLAN.md) · Tasks:
+[`../../TODO.md`](../../TODO.md).