Phase 6: fix the live Home Assistant write (Local Calendar, no event id)

The first live run against the new calendar.school entity failed loudly twice,
and both causes are fixed here.

- The Local Calendar rejects `return_response` on calendar.create_event. The
  client retries the call without it and ignores the changed-states array that
  Home Assistant returns instead.
- Home Assistant then reports no event id. The item keeps the description
  marker ([compass-nhs:<id>]) as its reference, so it leaves the candidate set
  and is never written twice. The live retry proved it: both events already in
  the calendar were adopted by their markers, with no duplicate.
- calendar.get_events reports `start` / `end` (a date-time or a bare date), not
  `start_date_time`; the reader now accepts all three shapes.

A model must no longer classify events. The calendar write depends on the
school-event rule, and the LLM labelled every event "normal"; events now keep
the deterministic rule under both classifiers.

Also fix the Phase 5 screenshot script: the calendar banner shares the `.scan`
class, so select each banner by its labelled section.

Docs record the accepted decision: Sam chose a Home Assistant Local Calendar,
so the target is calendar.school.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/parse/parse_test.go

@@ -302,6 +302,35 @@ func TestClassifyDeterministic(t *testing.T) {
 	}
 }
 
+// TestClassifyModelLeavesEventsDeterministic proves a model cannot label a
+// school event: the calendar write depends on that call (FR-8).
+func TestClassifyModelLeavesEventsDeterministic(t *testing.T) {
+	at := time.Date(2026, 10, 15, 18, 0, 0, 0, time.UTC)
+	items := []Item{
+		{Type: TypeEvent, Title: "End of Year Senior Music Concert - HALL - KSH", DueAt: &at},
+		{Type: TypeEvent, Title: "5 - 11HLEGB5 - D8 - LHT", DueAt: &at},
+		{Type: TypeTask, Title: "Essay", DueAt: &at},
+	}
+	reply, _ := json.Marshal([]map[string]any{
+		{"i": 0, "importance": "low"},
+		{"i": 1, "importance": "high"},
+		{"i": 2, "importance": "low"},
+	})
+	c := &stubCompleter{reply: string(reply)}
+	if err := ClassifyModel(context.Background(), c, items, "test-model", at); err != nil {
+		t.Fatalf("ClassifyModel: %v", err)
+	}
+	if items[0].Importance != importanceHigh {
+		t.Errorf("school event = %q, want %q", items[0].Importance, importanceHigh)
+	}
+	if items[1].Importance != importanceNormal {
+		t.Errorf("class period = %q, want %q", items[1].Importance, importanceNormal)
+	}
+	if items[2].Importance != importanceLow {
+		t.Errorf("task = %q, want the model's %q", items[2].Importance, importanceLow)
+	}
+}
+
 func ptr(t time.Time) *time.Time { return &t }
 
 // TestClassifyDeterministicEvents proves the Phase 6 importance rule: a school