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_crypt_valdris.go

@@ -14,9 +14,6 @@ package plugin
 //
 // Two paths to the boss (main_hall vs side_chapel); secret_chamber
 // dangles off side_chapel for an extra Perception gate with loot.
-// Registration is unconditional — the runtime gate
-// (GOGOBEE_BRANCHING_ZONES) decides whether new runs adopt this graph
-// or stay on the legacy linear template.
 
 func zoneCryptValdrisGraph() ZoneGraph {
 	nodes := []ZoneNode{