coach: keep cancelled-turn markers out of the plan fold too

The facts fold already skips the empty assistant row a cancelled turn
leaves behind; the plan fold read it as a blank turn and spent one of
its recent-turn slots on nothing. Same skip, same reason. Also state
the accepted one-slot slack a late rate-limit refund can leave in a
freshly opened window. Refs #781.

src/lib/ai/coach/plans.ts

@@ -276,6 +276,9 @@ async function loadRecentTurns(
   for (const m of [...row.messages].reverse()) {
     const content = decryptOrNull(m.encryptedContent);
     if (content === null) continue;
+    // #781 — skip empty bodies: a cancelled-turn marker (an empty assistant
+    // row closing an aborted turn) carries nothing to extract from.
+    if (!content.trim()) continue;
     turns.push({ role: m.role, content });
   }
   return turns;

src/lib/rate-limit.ts

@@ -61,6 +61,12 @@ export async function checkRateLimit(
  * once `reset_at` has passed the counter is stale and a decrement would bleed
  * into the NEXT window's budget, so an expired row is left untouched. Never
  * drops below zero (concurrent refunds after a window flip are a no-op).
+ *
+ * Known slack: if the charge's window expires and a new request opens the
+ * NEXT window before a late refund lands, the refund decrements that fresh
+ * window instead — at most one extra slot there, only on failure paths that
+ * never reached the metered work. Accepted; carrying the charge's `reset_at`
+ * through every failure path is not worth one slot of slack.
  */
 export async function refundRateLimit(key: string): Promise<void> {
   await prisma.$executeRaw`