practice: the learner's creation note — optional free-form input carried to partitioner + generators (owner, 2026-09-29)

- dialog: optional 'Anything else?' textarea (typing-only, 2000-char mirror)
- API: body gains note? — trimmed, whitespace-only omitted, hard 400 over 2000
- pipeline: note persisted on the set and threaded to both agent stages
- agents: THE LEARNER'S NOTE prompt block (present only when set) + the
  weighed-never-owed stance pinned in both doctrines
- fix en route: doubled THE PARAMETERS heading in both prompts since slice 3
  (renderParams owns the heading; the literal blocks removed)

src/store/practice.ts

@@ -111,6 +111,7 @@ export function isProblemSet(value: unknown): value is ProblemSet {
 		typeof v.id === "string" &&
 		v.id.startsWith("ps_") &&
 		typeof v.title === "string" &&
+		(v.note === undefined || typeof v.note === "string") &&
 		typeof v.createdAt === "string" &&
 		isParams(v.params) &&
 		isStringArray(v.nodeIds) &&