docs(recurring): record the coherence rule, and catch up §10-11 to what shipped

- AGENTS.md §1: reworded the "no horizon" invariant now that a parameter
  literally named horizonMonths exists. Nothing is stored — ExpectedBand's
  horizon only widens how far one read-only walk looks before the result is
  thrown away — but the old wording read as a contradiction to the next
  reader. The no-materialisation rule itself is unchanged.
- DECISIONS.md: added the "archived mirrors endedFrom" entry that was
  missing, including the previously-undocumented corollary that a
  future-dated cancellation dims the row immediately even though the bill
  keeps projecting until endedFrom.
- docs/specs/2026-09-03-recurring-costs-design.md §10-11: updated to
  describe the shipped UI — the "Coming up" group, the Ends column, the two
  coherence checks that gate a coming-up row (X1, X3), and the tests that
  now cover all of it.

docs/DECISIONS.md

@@ -105,6 +105,17 @@ record.
   happened. Under projection-only this would not have held, and a dated series
   would have been required.
   → [`specs/2026-09-03-recurring-costs-design.md`](specs/2026-09-03-recurring-costs-design.md)
+- **`archived` MIRRORS "`endedFrom` is set"**, maintained in one place
+  (`setRecurringCostEndedFrom`) rather than two independent fields a caller
+  could desynchronise. `endedFrom` stops the projection; `archived` only dims
+  the row, the same way an archived post stays listed rather than
+  disappearing. Neither touches a past occurrence. One consequence is visible
+  and undocumented until now: dating a cancellation in the future sets
+  `archived` immediately, so a bill still live for months still renders
+  dimmed with a "restart" button while it keeps projecting rows right up to
+  `endedFrom`. Defensible under the rule as chosen, not a bug.
+  → [`specs/2026-09-03-recurring-costs-design.md`](specs/2026-09-03-recurring-costs-design.md),
+  `src/store/actions.ts`
 
 ## Design system