feat(coach): close the plan check-in loop on the Today rail

Every accepted coach plan now earns a gentle weekly check-in. Confirming a
plan (proposed → active) without an explicit review date defaults one to a
week out, so a plan is always reviewed, not only the coach's own experiments.

The daily digest emits a single `coach_checkin` rail item when a standing
plan's review has come due — its pinned review date, or a defaulted
activation + 7 days for plans that predate it, or the read-back state a
reviewed plan carries. Capped at one check-in a day across all plans
(earliest-due wins); the rest wait for a following day. It reads existing
plan state only — no fresh generation, nothing warms on mount, and it is
gated on the coach module.

The card offers three one-tap actions through the existing plan-lifecycle
route: keep it (re-arm for another week), adjust it (open the coach), or let
it go (a guilt-free, respected end — the calm inversion of a streak). An
ignored check-in sits quiet and stops resurfacing after about two cycles;
its status is never changed behind the user's back.

Copy lands in all six locales.

src/hooks/use-coach-checkin.ts

@@ -0,0 +1,59 @@
+"use client";
+
+/**
+ * S3 — the coach check-in card's keep / let-go actions (§2.3).
+ *
+ * The Today rail surfaces a `coach_checkin` `PriorityItem` whose two mutating
+ * one-tap actions close the coach loop through the EXISTING plan-lifecycle
+ * route (`PATCH /api/coach/plans/[id]`) — no new backend surface:
+ *   - keep  → re-arm: status `active` + `reviewDate` pushed out one more cycle,
+ *             so the check-in comes back in a week rather than the same day;
+ *   - letGo → guilt-free retirement: status `abandoned`, a terminal, respected
+ *             outcome (the calm inversion of a streak). The card never nags.
+ *
+ * "Adjust" is navigation (an `href` into the coach) handled by `PriorityCard`
+ * as a `<Link>` — it never reaches this hook.
+ *
+ * Both invalidate the daily digest (so the acted-on card leaves the rail) and
+ * every plans-list slot (so the /coach/plans ledger stays in step). Keys route
+ * through the centralised factory — no bare arrays.
+ */
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+
+import { apiPatch } from "@/lib/api/api-fetch";
+import { queryKeys } from "@/lib/query-keys";
+import { COACH_CHECKIN_REVIEW_DAYS } from "@/lib/daily/digest";
+
+export function useCoachCheckinAction() {
+  const queryClient = useQueryClient();
+  const invalidate = () => {
+    void queryClient.invalidateQueries({ queryKey: queryKeys.dailyDigest() });
+    void queryClient.invalidateQueries({ queryKey: queryKeys.coachPlansAll() });
+  };
+
+  const keep = useMutation({
+    mutationFn: async (planId: string) => {
+      const reviewDate = new Date(
+        Date.now() + COACH_CHECKIN_REVIEW_DAYS * 86_400_000,
+      ).toISOString();
+      await apiPatch(`/api/coach/plans/${encodeURIComponent(planId)}`, {
+        status: "active",
+        reviewDate,
+      });
+      return planId;
+    },
+    onSuccess: invalidate,
+  });
+
+  const letGo = useMutation({
+    mutationFn: async (planId: string) => {
+      await apiPatch(`/api/coach/plans/${encodeURIComponent(planId)}`, {
+        status: "abandoned",
+      });
+      return planId;
+    },
+    onSuccess: invalidate,
+  });
+
+  return { keep, letGo };
+}