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_zone_combat.go

@@ -130,13 +130,10 @@ func (p *AdventurePlugin) runZoneCombat(
 
 	playerStats, playerMods := DerivePlayerStats(char, equip, bonuses, chatLvl, char.CurrentStreak, false)
 
-	dndChar, freshMigrate, err := ensureDnDCharacterForCombat(userID, char)
+	dndChar, _, err := ensureDnDCharacterForCombat(userID, char)
 	if err != nil {
 		return CombatResult{}, fmt.Errorf("ensure dnd character: %w", err)
 	}
-	if freshMigrate {
-		p.maybeSendDnDOnboarding(userID, char, dndChar)
-	}
 	applyDnDPlayerLayer(&playerStats, dndChar)
 	applyDnDEquipmentLayer(&playerStats, dndChar, equip)
 	applyDnDHPScaling(&playerStats, dndChar)