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_setup.go

@@ -383,6 +383,10 @@ func (p *AdventurePlugin) handleDnDRespecCmd(ctx MessageContext) error {
 	c.PendingSetup = true
 	c.AutoMigrated = false
 	c.LastRespecAt = &now
+	// Phase 10: full wipe also resets subclass state. Subclass-respec
+	// cooldown is dropped — at L1 with no class there's nothing to gate.
+	c.Subclass = ""
+	c.LastSubclassRespecAt = nil
 	// Save the wipe BEFORE debiting euros (audit fix B). If save fails, the
 	// player's old state survives and they keep their euros — better than
 	// a destructive debit-without-wipe.