Adv 2.0 L5 close-out: loader rewire + adventure_characters purge gate

Rewires loadAdvCharacter / createAdvCharacter to source from player_meta,
drops every legacy-table fallback in the loadXxxState helpers, and adds a
GOGOBEE_LEGACY_PURGE=1 gate to drop the now-cold adventure_characters
table. Schema CREATE removed and column migrations tolerate the missing
table so the purge stays sticky across reboots.

§7.4 of the migration plan is reconciled to document that
player_meta.combat_level stays — combat_stats.go consumes it via the
overlay and is shared infrastructure, not legacy.

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

internal/plugin/cleanup_l5.go

@@ -0,0 +1,40 @@
+package plugin
+
+import (
+	"log/slog"
+	"os"
+
+	"gogobee/internal/db"
+)
+
+// purgeLegacyAdvCharacterTable drops the `adventure_characters` table when
+// the operator opts in via GOGOBEE_LEGACY_PURGE=1. Phase L5 close-out: the
+// table has been frozen since L5h (saveAdvCharacter fans out into player_meta
+// only), and the loader rewire moved every read off it. This is the final
+// step.
+//
+// The pre-D&D rollback snapshot table (`adventure_characters_pre_dnd`) is
+// preserved — it's a separate insurance copy and a future
+// `GOGOBEE_PRE_DND_PURGE=1` pass can drop it once the soak window expires.
+//
+// Idempotent: skips silently if the table is already gone (or never existed
+// on a fresh install).
+func purgeLegacyAdvCharacterTable() {
+	if os.Getenv("GOGOBEE_LEGACY_PURGE") != "1" {
+		return
+	}
+	d := db.Get()
+	var name string
+	err := d.QueryRow(
+		`SELECT name FROM sqlite_master WHERE type='table' AND name='adventure_characters'`,
+	).Scan(&name)
+	if err != nil {
+		// sql.ErrNoRows or any scan error → nothing to drop.
+		return
+	}
+	if _, err := d.Exec(`DROP TABLE adventure_characters`); err != nil {
+		slog.Error("L5 purge: drop adventure_characters failed", "err", err)
+		return
+	}
+	slog.Warn("L5 purge: adventure_characters table dropped (GOGOBEE_LEGACY_PURGE=1)")
+}