Adv 2.0 D&D Phase 10 SUB2a-i: Champion + Berserker + Thief L5/L7

First mechanical slice of the subclass system. Each subclass gets its L5
and L7 abilities wired through the existing combat engine + skill check
substrate; no new resource pools introduced.

Champion (L5/L7):
- Improved Critical: new Mods.CritThreshold lowers crit floor to nat 19+
  in resolvePlayerAttack. Default 20 preserved for non-Champions.
- Remarkable Athlete: +½ proficiency bonus to STR/DEX/CON skill checks
  via subclassSkillBonus, layered on raceSkillBonus.

Berserker (L5/L7):
- Rage: new !arm-able active ability gated to Berserker subclass via
  DnDAbility.Subclass field. Reuses the Fighter stamina pool (3/long-rest
  matches 5e's rage uses). On fire: +2 flat damage per hit, halve incoming
  weapon damage, +50% damage approximation for Frenzy's bonus-attack-per-
  turn (one-shot combat can't model that literally).
- Frenzy exhaustion: new exhaustion column on dnd_character; incremented
  by persistDnDPostCombatSubclass after any rage'd combat. Decremented by
  one per long rest.
- Mindless Rage: documented; charm/frighten conditions don't exist in the
  combat engine yet, so no functional effect until P11 zone bosses land.

Thief (L5/L7):
- Sleight of Hand added to dndSkillTable.
- Fast Hands: +5 to Sleight of Hand checks via subclassSkillBonus.
- Supreme Sneak: advantage on Stealth (best of two d20s) via new
  subclassSkillAdvantage hook in performSkillCheck. 5e's half-speed gate
  dropped — we don't track movement speed.

Plumbing:
- applySubclassPassives layered after applyClassPassives in both arena
  and dungeon combat bridges.
- DnDAbility.Subclass enables per-subclass gating; characterActiveAbilities
  filters !arm and !abilities lists by both class and subclass.
- Existing !respec full-wipe also clears Exhaustion.

Tests: Champion CritThreshold gating across L4/L5/no-subclass; rage Apply
flag-set; rage probabilistic win-rate lift vs tougher enemy; exhaustion
increment only on raged combats; long-rest decrement; Champion bonus on
STR/DEX/CON only and only at L7+; Thief Fast Hands SoH-only; Thief
Supreme Sneak L7+ Stealth-only; statistical advantage roll lifts Thief
stealth average by ≥2 vs plain Rogue; !arm rage subclass gating; schema
roundtrip; characterActiveAbilities visibility.

internal/plugin/dnd_sheet.go

@@ -23,8 +23,8 @@ func (p *AdventurePlugin) handleDnDAbilitiesCmd(ctx MessageContext) error {
 	b.WriteString(fmt.Sprintf("**Class passive — %s**\n  %s\n\n", ab.Name, ab.Description))
 	b.WriteString(fmt.Sprintf("**Race trait**\n  %s\n", ri.Passive))
 
-	// Active abilities (Phase 6)
-	actives := classActiveAbilities(c.Class)
+	// Active abilities (Phase 6 + Phase 10 subclass-gated)
+	actives := characterActiveAbilities(c)
 	if len(actives) > 0 {
 		resType, _ := classResourceMax(c.Class)
 		cur, max, _ := getResource(c.UserID, resType)