Adv 2.0 L5 close-out: §7.4 reconciled + retire 3 CombatLevel readers

Reconcile the migration plan with what actually shipped, and clear the
three remaining CombatLevel reader sites so the §7 grep gates hold.

Plan reconciliation (gogobee_legacy_migration.md §7.4):
The original deletion list was wrong on two counts. (1) AdventureCharacter
struct survives as the loaded-view shape post-L5h overlay; only the
backing table drops at purge. (2) combat_engine.go / combat_bridge.go /
combat_stats.go are not legacy — they're the live combat engine the D&D
system layers on top of (applyDnDPlayerLayer/EquipmentLayer/etc. mutate
CombatStats before SimulateCombat). Verified via grep: 21+ files reference
these symbols across arena/dungeon/zone/expedition. §7.4 rewritten to
reflect this; no L6 combat-engine migration needed.

Reader fixes:
- babysitDailyCost reframed for D&D-level scale. Formula 100 + level*100
  preserves the curve at every old-CL boundary given the 5:1 compression
  in dndLevelFromCombatLevel (Level 4 ≈ old CL 20 = €500/day, Level 10 ≈
  old CL 50 = €1100/day). Caller switched to dndLevelForUser.
- dnd_sheet.go drops the vestigial "Combat (legacy) %d" stat-block line.
- D&D onboarding retired. Post-L5g the welcome DM never fires (every
  legacy player already has a D&D row), so dnd_onboarding.go +
  dnd_onboarding_test.go are deleted, the 3 production caller invocations
  (dnd_zone_combat, combat_bridge, dnd_combat::ensureCharForDnDCmd) and
  the dnd_setup.go stub-creation branch removed. OnboardingSent column
  kept for a separate cleanup pass. dnd_audit_fixes_test.go ported.

go vet ./... && go test ./... clean.

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

internal/plugin/dnd_setup.go

@@ -116,34 +116,6 @@ func (p *AdventurePlugin) dndSetupStatus(ctx MessageContext) error {
 
 	// No row yet → fresh setup. Show suggestion + race menu.
 	if c == nil {
-		// Legacy-player welcome: if they're meeting D&D for the first time
-		// via `!setup` (rather than via combat auto-migration), they still
-		// deserve the onboarding DM. We persist a stub draft row so the
-		// OnboardingSent flag carries forward — neither this nor any later
-		// auto-migration will re-send the welcome.
-		advChar, _ := loadAdvCharacter(ctx.Sender)
-		if advChar != nil && advChar.CombatLevel >= dndLegacyMinLevel {
-			now := time.Now().UTC()
-			// Seed the stub's Level with the computed mapping so the welcome DM
-			// reports the right number ("your level X is now Adv 2.0 level Y").
-			// On !setup confirm the level is recomputed from the same formula,
-			// so the value here matches what the player will actually end up with.
-			stub := &DnDCharacter{
-				UserID:         ctx.Sender,
-				Level:          dndLevelFromCombatLevel(advChar.CombatLevel),
-				ArmorClass:     10,
-				PendingSetup:   true,
-				OnboardingSent: false, // maybeSendDnDOnboarding will flip this on success
-				CreatedAt:      now,
-				UpdatedAt:      now,
-			}
-			if err := SaveDnDCharacter(stub); err != nil {
-				slog.Error("dnd: setup stub save failed", "user", ctx.Sender, "err", err)
-			} else {
-				p.maybeSendDnDOnboarding(ctx.Sender, advChar, stub)
-			}
-		}
-
 		sug := inferDnDFromArchetypes(ctx.Sender)
 		var b strings.Builder
 		b.WriteString("⚔️ **Adv 2.0 Setup** — let's build your character.\n\n")