fix: address verified gadfly P5 findings (canary robustness)

All 3 cloud models converged (all "minor" — example code, no blocking):
- Consolidate: a model whose every lens errored now reads "review incomplete",
  not a misleading "no issues found" (all 3 models). + test.
- Consolidate: swarm-cancelled (unattributed) cells now surface a "swarm
  cancelled — N cell(s) did not run" banner instead of vanishing (all 3). + test.
- main: io.ReadAll(os.Stdin) error is surfaced (all 3); a TTY stdin no longer
  hangs forever (TTY guard, minimax).
- providerOf: a bare tier name now keys its own PerKey bucket instead of all
  bare tiers collapsing onto "tier" (minimax, glm-5.2) — distinct tiers throttle
  independently.
- Review doc reworded (the closure, not fanout, carries per-cell errors).

Left as documented example-scope behavior: no per-cell timeout (caller supplies
ctx), unknown-severity → lowest rank (no crash).

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

examples/reviewer/reviewer_test.go

@@ -100,3 +100,29 @@ func TestConsolidateVerdicts(t *testing.T) {
 		t.Errorf("critical + errored lens header wrong:\n%s", got)
 	}
 }
+
+// TestConsolidateAllErrored: a model whose every lens errored must NOT be
+// labelled "no issues found" (the gadfly P5 finding).
+func TestConsolidateAllErrored(t *testing.T) {
+	got := Consolidate([]LensResult{
+		{Model: "m", Lens: "a", Err: context.DeadlineExceeded},
+		{Model: "m", Lens: "b", Err: context.DeadlineExceeded},
+	})
+	if !strings.Contains(got, "m — review incomplete") {
+		t.Errorf("all-errored model should be 'review incomplete', got:\n%s", got)
+	}
+	if strings.Contains(got, "no issues found") {
+		t.Errorf("all-errored model must not say 'no issues found':\n%s", got)
+	}
+}
+
+// TestConsolidateSwarmCancelled: dropped (unattributed) cells surface a banner.
+func TestConsolidateSwarmCancelled(t *testing.T) {
+	got := Consolidate([]LensResult{
+		{Err: context.Canceled}, // dropped cell, no model
+		{Model: "m", Lens: "a", Findings: []Finding{{Severity: SevSmall, Title: "x"}}},
+	})
+	if !strings.Contains(got, "swarm cancelled") {
+		t.Errorf("dropped cells should surface a cancellation banner:\n%s", got)
+	}
+}