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_map.go

@@ -2,12 +2,12 @@ package plugin
 
 // Phase G6 — graph-aware !zone map renderer.
 //
-// Replaces the linear renderZoneMap() output when GOGOBEE_BRANCHING_ZONES
-// is on. Walks the registered (or legacy-compiled) ZoneGraph in BFS
-// order rooted at Entry, lays nodes out by PosX/PosY, and prints a
-// horizontal glyph row per layer with a status row underneath. Locked
-// outgoing edges show as `╳`; secret nodes stay hidden until the
-// player has visited them (prevents spoilers from `!zone map`).
+// Replaces the linear renderZoneMap() output. Walks the registered
+// (or legacy-compiled) ZoneGraph in BFS order rooted at Entry, lays
+// nodes out by PosX/PosY, and prints a horizontal glyph row per layer
+// with a status row underneath. Locked outgoing edges show as `╳`;
+// secret nodes stay hidden until the player has visited them
+// (prevents spoilers from `!zone map`).
 
 import (
 	"fmt"