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:`.

backend/app/sync/tagsuggest.py

@@ -12,7 +12,7 @@ result (measured on real data: every untagged channel got a suggestion, most on
 alone). Topic overlap is what makes "you tagged similar channels this way" meaningful.
 
 Purely read-only and deterministic — the Channel Manager surfaces the result as one-tap-accept
-"ghost" chips; nothing is written here (D4 = suggest-and-confirm, no silent auto-apply). The
+"ghost" chips; nothing is written here (suggest-and-confirm, never a silent auto-apply). The
 thresholds are tuning constants rather than admin knobs on purpose: they mirror how ``autotag.py``
 keeps its mappings inline, and sysconfig only stores int/str/bool (no float), so a knob would mean
 storing percentages — not worth the surface for values an operator has no reason to tune per-install.