Adv 2.0 D&D Phase 10 SUB1: subclass selection system

Implements the identity layer of gogobee_subclass_system.md: 15 subclasses
(3 per class) selectable at L5 via !subclass.

- Schema: new subclass + last_subclass_respec_at columns on dnd_character.
- Registry (dnd_subclass.go): 15 entries with class, display, tagline, and
  TwinBee flavor line; helpers for prompt rendering and input resolution
  (number, id, or display-name; case/space/hyphen-insensitive).
- !subclass command: bare form prints prompt or current; !subclass <name>
  selects (free first time) or changes (500 euros + 30-day cooldown via
  separate timestamp from the full !respec wipe). Save-then-debit ordering
  mirrors !respec audit fix B.
- Level-up DM at L5+ now embeds the real selection prompt instead of the
  Phase 9 placeholder; suppressed once a subclass is set.
- Sheet shows the subclass below the class line, or nudges !subclass when
  unchosen at L5+.
- Existing !respec full-wipe also clears subclass + cooldown timestamp.

Mechanical effects of L5/7/10/15 subclass abilities deferred to SUB2/SUB3.

internal/plugin/dnd_sheet.go

@@ -77,6 +77,13 @@ func renderDnDSheet(c *DnDCharacter, adv *AdventureCharacter, equip map[Equipmen
 		name = adv.DisplayName
 	}
 	b.WriteString(fmt.Sprintf("⚔️ **%s** — Level %d %s %s\n", name, c.Level, ri.Display, ci.Display))
+	if c.Subclass != "" {
+		if si, ok := subclassInfo(c.Subclass); ok {
+			b.WriteString(fmt.Sprintf("    _%s_\n", si.Display))
+		}
+	} else if c.Level >= dndSubclassMinLevel {
+		b.WriteString("    _Subclass: unchosen — run `!subclass`_\n")
+	}
 	b.WriteString(strings.Repeat("─", 36) + "\n")
 
 	// Vitals