docs: restore single-YYYY-MM-DD header Date per README convention (PR #817 r2)

Codex P2 on PR #817. The r1 header carried two values
("2026-05-22 (proposed) -> 2026-05-24 (implemented)") which
conflicted with the docs/design/README.md convention that the
header Date matches the filename propose date — one canonical
YYYY-MM-DD value, with the rest of the lifecycle living inside
the document body.

Fix: Date: line reverts to 2026-05-22 (matches the filename
propose date prefix). The implemented date moves into the
Implementation record paragraph, where the per-phase PR table
already carries enough context to date the milestones.

Author: bootjp

docs/design/2026_05_22_implemented_admin_data_browser.md

@@ -2,13 +2,14 @@
 
 **Status:** Implemented
 **Author:** bootjp
-**Date:** 2026-05-22 (proposed) → 2026-05-24 (implemented)
+**Date:** 2026-05-22
 
 ---
 
 ## Implementation record
 
-All six phases shipped between 2026-05-22 and 2026-05-24:
+Final milestone landed 2026-05-24. All six phases shipped between
+2026-05-22 and 2026-05-24:
 
 | Phase | What landed | PR |
 |-------|-------------|----|