Branching zones G9c: drop legacy columns + add purge gate

Final G9 step. The schema no longer creates current_room or
room_seq_json on fresh installs, and existing prod schemas drop them
via purgeLegacyZoneRunColumns() once the operator flips
GOGOBEE_BRANCHING_PURGE=1.

- CREATE TABLE dnd_zone_run loses both columns; comment updated to
  point at G1's columnMigrations for the graph-mode columns.
- cleanup_g9.go mirrors the L5 idempotent-purge shape: ALTER TABLE
  DROP COLUMN per legacy column, "no such column" treated as already
  done, default-off.
- Wired into adventure Init alongside purgeLegacyAdvCharacterTable.
- Plan doc marks G9 complete.

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

internal/plugin/adventure.go

@@ -200,6 +200,10 @@ func (p *AdventurePlugin) Init() error {
 	// Phase L5 close-out — drop the now-cold adventure_characters table when
 	// the operator sets GOGOBEE_LEGACY_PURGE=1. Default off; idempotent.
 	purgeLegacyAdvCharacterTable()
+	// Phase G9 close-out — drop the now-cold current_room / room_seq_json
+	// columns from dnd_zone_run when the operator sets
+	// GOGOBEE_BRANCHING_PURGE=1. Default off; idempotent.
+	purgeLegacyZoneRunColumns()
 	// Phase R1 orphan-archive used to run here on every Init, but it
 	// over-archived: it treats any active dnd_zone_run row not linked to
 	// an active expedition as a legacy `!adventure dungeon` orphan, which

internal/plugin/cleanup_g9.go

@@ -0,0 +1,37 @@
+package plugin
+
+import (
+	"log/slog"
+	"os"
+	"strings"
+
+	"gogobee/internal/db"
+)
+
+// purgeLegacyZoneRunColumns drops the dnd_zone_run.current_room and
+// dnd_zone_run.room_seq_json columns when the operator opts in via
+// GOGOBEE_BRANCHING_PURGE=1. Phase G9 close-out: G9a removed the
+// runtime gate, G9b stopped persisting the columns, and this is the
+// final step.
+//
+// Idempotent: skips silently when the columns are already gone (or
+// never existed on a fresh install — the schema in db.go no longer
+// creates them).
+func purgeLegacyZoneRunColumns() {
+	if os.Getenv("GOGOBEE_BRANCHING_PURGE") != "1" {
+		return
+	}
+	d := db.Get()
+	for _, col := range []string{"current_room", "room_seq_json"} {
+		if _, err := d.Exec("ALTER TABLE dnd_zone_run DROP COLUMN " + col); err != nil {
+			msg := err.Error()
+			// "no such column" → already dropped (or fresh install).
+			if strings.Contains(msg, "no such column") {
+				continue
+			}
+			slog.Error("G9 purge: drop column failed", "col", col, "err", err)
+			return
+		}
+		slog.Warn("G9 purge: dnd_zone_run column dropped (GOGOBEE_BRANCHING_PURGE=1)", "col", col)
+	}
+}