fix(gate): the plan-code guard catches decision codes, and the purge takes them

A plan's decision ids (`D1`, `D4`, `D6-A`) cited in 11 comments and test titles named a document
no reader of this repo has, and the guard had no pattern for them. It has now (mirrored in
audit_metrics), with cases for each form the citations took and for the look-alikes it must leave
alone (a JPEG marker, "3D"); the 11 lines say their why without the id. Case test seen red without
the pattern, the scan red on a planted `D9:`.

frontend/src/components/plexCards.dom.test.tsx

@@ -258,7 +258,7 @@ describe("EpisodeCard a11y", () => {
 });
 
 // In a keyboard-walkable grid or rail the WHOLE card is the item (the ring goes round the
-// poster and its title, D2) and the poster — the card's own role=button — is what Enter opens.
+// poster and its title) and the poster — the card's own role=button — is what Enter opens.
 describe("Plex cards in a keyboard-walkable list", () => {
   const cases = [
     [