fix(gate): the plan-code guard catches bare sprint and phase codes, and the purge takes the P ones

The sprint alternative matched only `(S6)` or `S3e2`, so the commonest purged form (`# S6:`,
`S3's`, `"""S6 —`, `S1 justified`) and every phase code (`P1`) would come back unflagged; the phase
codes had in fact survived the purge in 16 lines (Plex client sections, the upload's header shape),
now reworded around them.

One pattern now covers epic, roadmap, sprint and phase codes; the named exceptions are TV episode
notation, a fixture's season or episode title (`title="S1"`) and ruff's `E4/E7/E9`, and a ruff rule
like `S110` does not match. The case test lists the purged forms and no longer claims every form.
audit_metrics copies the definition (ASCII, as JavaScript's \b is) and reports it as
plan_code_lines / plan_code_files.

frontend/src/components/PlexCollectionEditor.tsx

@@ -14,7 +14,7 @@ import { Pressable } from "./ui/Pressable";
 import { Input } from "./ui/Input";
 import { SURFACE } from "./ui/surfaces";
 
-// Admin-only "manage this movie's collections" dialog (Collections P2). Lists the library's EDITABLE
+// Admin-only "manage this movie's collections" dialog. Lists the library's EDITABLE
 // collections with an in/out toggle (add/remove the movie, written back to Plex), plus a field to
 // create a new collection seeded with this movie. Read-only collections (smart / IMDb-TMDb auto-lists)
 // never appear here. Opened from the movie info page.