memory: reconcile timer memories with Syncro add_line_item switch

Mike's overhaul replaced the timer workflow with add_line_item, and he
already rewrote feedback_syncro_timer_first.md. Reconcile the leftovers:
- MEMORY.md index line for timer_first still stated the superseded
  "timers required" rule as current — rewrite to the add_line_item rule.
- timer_entry response-shape memory is now about a dead workflow — mark
  it HISTORICAL (index + file banner), retained only for manual timer use.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/memory/feedback_syncro_timer_response_shape.md

@@ -4,6 +4,10 @@ description: POST /tickets/{id}/timer_entry returns a flat object {"id": N, "tic
 type: feedback
 ---
 
+> **SUPERSEDED / HISTORICAL — 2026-05-21.** Timers are no longer part of the ACG Syncro
+> billing workflow; billing uses `add_line_item` directly. See [[Syncro — use add_line_item for billing, not timers]] (`feedback_syncro_timer_first.md`). Keep this note ONLY as reference
+> for the rare case a timer is created manually — do not treat it as current workflow.
+
 **Rule:** When parsing the response from `POST /tickets/{id}/timer_entry`, use `.id` directly — the response is a FLAT object. Do NOT use `.timer.id // .timer_entry.id`.
 
 **Verified response shape (2026-05-05, ticket #32253):**