feat(report): Stage-0 usage gate counts from a baseline date (default 2026-06-11)

The return-usage gate (ADR-016) counted distinct active weeks over ALL history,
so pre-launch noise — testing churn and the period the pilot sat blocked on zero
summaries — would inflate the signal. Add a baseline: ActiveWeeks(ctx, since)
filters login_events + summary_actions to seen_at/acted_at >= since. The report
command sets it to TAPIR_USAGE_GATE_START (YYYY-MM-DD, default 2026-06-11 — the
morning the pilot was unblocked) and prints the baseline. The gate now measures
whether users RETURN once it genuinely works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/tapir/report.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"text/tabwriter"
+	"time"
 
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
 )
@@ -14,6 +15,27 @@ import (
 // weeks. The gate passes when any user reaches it.
 const gateThreshold = 2
 
+// defaultGateStart is the date Stage-0 return-usage tracking begins: the morning
+// the pilot was actually unblocked and summaries started flowing (2026-06-11).
+// Activity before this — testing, the period the pilot was stuck on zero — is
+// noise and must not count toward the gate. Override with TAPIR_USAGE_GATE_START
+// (YYYY-MM-DD). The gate measures whether users RETURN once it genuinely works.
+const defaultGateStart = "2026-06-11"
+
+// gateStart resolves the baseline date from TAPIR_USAGE_GATE_START or the default,
+// parsed as a UTC calendar day.
+func gateStart() (time.Time, error) {
+	v := os.Getenv("TAPIR_USAGE_GATE_START")
+	if v == "" {
+		v = defaultGateStart
+	}
+	t, err := time.Parse("2006-01-02", v)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("TAPIR_USAGE_GATE_START=%q: want YYYY-MM-DD: %w", v, err)
+	}
+	return t, nil
+}
+
 // runReport prints the Stage-0 usage gate: per-user distinct active weeks (reads
 // UNION acts) and the pass/fail verdict. Read-only, cross-user — needs only
 // TAPIR_DB_DSN (not TAPIR_USER_ID; the report enumerates all users itself).
@@ -28,16 +50,24 @@ func runReport(ctx context.Context, _ []string) error {
 	}
 	defer s.Close()
 
-	rows, err := s.ActiveWeeks(ctx)
+	since, err := gateStart()
 	if err != nil {
 		return err
 	}
-	return formatReport(os.Stdout, rows)
+
+	rows, err := s.ActiveWeeks(ctx, since)
+	if err != nil {
+		return err
+	}
+	return formatReport(os.Stdout, rows, since)
 }
 
 // formatReport renders the per-user week counts and the gate verdict. Pure: no DB,
 // no env — so the layout and verdict logic are unit-testable without Postgres.
-func formatReport(w io.Writer, rows []store.UserActiveWeeks) error {
+func formatReport(w io.Writer, rows []store.UserActiveWeeks, since time.Time) error {
+	if _, err := fmt.Fprintf(w, "Counting usage since %s (Stage-0 gate baseline)\n\n", since.Format("2006-01-02")); err != nil {
+		return err
+	}
 	if len(rows) == 0 {
 		_, err := fmt.Fprintln(w, "no users yet")
 		return err