Branching zones G9a: remove GOGOBEE_BRANCHING_ZONES gate

All 9 zones have hand-authored graphs (G8a–G8i), so the POC gate has
served its purpose. Make graph mode the only runtime path:

- Drop branchingZonesEnabled() and the os import in zone_graph_nav.go.
- Inline the gate-on branches in zoneCmdMap, zoneCmdAdvance,
  CurrentRoomType, startZoneRun, and the expedition map renderer.
- Keep the legacy linear functions (markRoomCleared, renderZoneMap,
  generateRoomSequence) standing — their tests still exercise them and
  they'll retire alongside current_room / room_seq_json in G9b.
- Strip gate test (TestBranchingZonesGate, TestCurrentRoomType_GateOff)
  and t.Setenv calls from the surviving graph tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/zone_graph_nav_test.go

@@ -241,17 +241,3 @@ func TestNodeKindToRoomType(t *testing.T) {
 	}
 }
 
-func TestBranchingZonesGate(t *testing.T) {
-	t.Setenv("GOGOBEE_BRANCHING_ZONES", "")
-	if branchingZonesEnabled() {
-		t.Error("gate should be off when env empty")
-	}
-	t.Setenv("GOGOBEE_BRANCHING_ZONES", "1")
-	if !branchingZonesEnabled() {
-		t.Error("gate should be on when env=1")
-	}
-	t.Setenv("GOGOBEE_BRANCHING_ZONES", "0")
-	if branchingZonesEnabled() {
-		t.Error("gate should be off when env=0")
-	}
-}