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_subclass_cmd.go

@@ -0,0 +1,145 @@
+package plugin
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// !subclass — Phase 10 SUB1.
+//
+// Usage:
+//
+//	!subclass                  → show selection prompt for player's class
+//	!subclass <name|number>    → choose / change subclass
+//
+// Rules:
+//   - L<5: blocked with hint message.
+//   - First selection (Subclass == ""): free, no cooldown.
+//   - Subsequent change: 30-day cooldown + 500-coin cost. Same surface
+//     command — no separate `respec` subcommand. The full-character-wipe
+//     `!respec` command is unchanged.
+//
+// Mechanical effects of L5/7/10/15 subclass abilities arrive in SUB2/SUB3.
+
+const (
+	dndSubclassMinLevel       = 5
+	dndSubclassRespecCost     = 500
+	dndSubclassRespecCooldown = 30 * 24 * time.Hour
+)
+
+func (p *AdventurePlugin) handleDnDSubclassCmd(ctx MessageContext, args string) error {
+	userMu := p.advUserLock(ctx.Sender)
+	userMu.Lock()
+	defer userMu.Unlock()
+
+	c, err := LoadDnDCharacter(ctx.Sender)
+	if err != nil {
+		return p.SendDM(ctx.Sender, "Couldn't load your character: "+err.Error())
+	}
+	if c == nil || c.PendingSetup {
+		return p.SendDM(ctx.Sender,
+			"No Adv 2.0 character yet — run `!setup` (or just enter combat and we'll auto-build one).")
+	}
+	if c.Level < dndSubclassMinLevel {
+		return p.SendDM(ctx.Sender, fmt.Sprintf(
+			"Subclass selection unlocks at **Level %d**. You're currently L%d.",
+			dndSubclassMinLevel, c.Level))
+	}
+	if len(subclassesForClass(c.Class)) == 0 {
+		return p.SendDM(ctx.Sender, "Your class has no subclasses registered yet.")
+	}
+
+	args = strings.TrimSpace(args)
+
+	// Bare `!subclass` → prompt or current.
+	if args == "" {
+		if c.Subclass == "" {
+			return p.SendDM(ctx.Sender, renderSubclassPrompt(c.Class))
+		}
+		info, _ := subclassInfo(c.Subclass)
+		var b strings.Builder
+		b.WriteString(fmt.Sprintf("**Current subclass:** %s — _%s_\n\n", info.Display, info.Tagline))
+		b.WriteString(fmt.Sprintf(
+			"To change: `!subclass <name>` — costs **%d euros**, **%d-day** cooldown.\n",
+			dndSubclassRespecCost, int(dndSubclassRespecCooldown/(24*time.Hour))))
+		return p.SendDM(ctx.Sender, b.String())
+	}
+
+	// `!subclass <name|number>` → set or change.
+	chosen, ok := resolveSubclassInput(c.Class, args)
+	if !ok {
+		var b strings.Builder
+		b.WriteString("Unknown subclass for your class. Options:\n\n")
+		b.WriteString(renderSubclassPrompt(c.Class))
+		return p.SendDM(ctx.Sender, b.String())
+	}
+
+	// First selection — free.
+	if c.Subclass == "" {
+		return p.applySubclassChoice(ctx, c, chosen, false)
+	}
+
+	// Same subclass already chosen — no-op.
+	if c.Subclass == chosen {
+		info, _ := subclassInfo(chosen)
+		return p.SendDM(ctx.Sender, fmt.Sprintf(
+			"You're already a **%s**. No change made.", info.Display))
+	}
+
+	// Change — gated by cooldown + cost.
+	if c.LastSubclassRespecAt != nil {
+		elapsed := time.Since(*c.LastSubclassRespecAt)
+		if elapsed < dndSubclassRespecCooldown {
+			return p.SendDM(ctx.Sender, fmt.Sprintf(
+				"Subclass change is on cooldown — %s remaining.",
+				formatRespecDuration(dndSubclassRespecCooldown-elapsed)))
+		}
+	}
+	if p.euro == nil || p.euro.GetBalance(ctx.Sender) < float64(dndSubclassRespecCost) {
+		return p.SendDM(ctx.Sender, fmt.Sprintf(
+			"Changing subclass costs **%d euros** — you don't have enough.",
+			dndSubclassRespecCost))
+	}
+	return p.applySubclassChoice(ctx, c, chosen, true)
+}
+
+// applySubclassChoice writes the new subclass to the character and (if
+// charged) debits the cost. Saves before debiting (audit pattern from
+// !respec): if the save fails the player keeps their euros.
+func (p *AdventurePlugin) applySubclassChoice(
+	ctx MessageContext, c *DnDCharacter, chosen DnDSubclass, charged bool,
+) error {
+	prev := c.Subclass
+	c.Subclass = chosen
+	if charged {
+		now := time.Now().UTC()
+		c.LastSubclassRespecAt = &now
+	}
+	if err := SaveDnDCharacter(c); err != nil {
+		return p.SendDM(ctx.Sender, "Couldn't save subclass choice: "+err.Error())
+	}
+	if charged {
+		// Debit last; race vs. balance change is rare and strictly safer
+		// to favor the player than risk taking euros without applying the
+		// change (matches the !respec ordering).
+		// Race vs. balance change is rare; if Debit fails the save
+		// already landed. Mirrors the !respec ordering in dnd_setup.go.
+		_ = p.euro.Debit(ctx.Sender, float64(dndSubclassRespecCost), "dnd subclass change")
+	}
+
+	info, _ := subclassInfo(chosen)
+	var b strings.Builder
+	if prev == "" {
+		b.WriteString(fmt.Sprintf("🎯 **Subclass selected: %s**\n\n", info.Display))
+	} else {
+		prevInfo, _ := subclassInfo(prev)
+		b.WriteString(fmt.Sprintf("🎯 **Subclass changed: %s → %s**\n\n",
+			prevInfo.Display, info.Display))
+		b.WriteString(fmt.Sprintf("_%d euros spent. Cooldown: %dd before next change._\n\n",
+			dndSubclassRespecCost, int(dndSubclassRespecCooldown/(24*time.Hour))))
+	}
+	b.WriteString("_" + info.Flavor + "_\n\n")
+	b.WriteString("_(Mechanical L5/7/10/15 subclass abilities land in upcoming phases.)_")
+	return p.SendDM(ctx.Sender, b.String())
+}