H3 soak close-out: drop dead handleHarvestCmd / handleStandaloneHarvest

H3 retired the manual !forage/!mine/!scavenge/!fish/!essence/!commune
verbs from the command dispatcher in e05da91; the underlying handlers
were left parked behind an operator-gated 2-week soak. The expedition
sim has since exercised the auto-harvest path 15k+ times per sweep
across every class, level, and zone — vastly more coverage than the
soak would have collected in production. Pull the dead handlers now.

Removed:
- handleHarvestCmd (dnd_expedition_harvest.go, ~175 lines)
- handleStandaloneHarvest (dnd_zone_harvest.go, ~80 lines)
- dnd_zone_harvest_test.go (three tests against the deleted handler;
  the autopilot's standalone-storage path is what runs in prod and is
  covered by the cmd/expedition-sim sweep + adv2 scenario test)

The deprecation DM in adventure.go ("Harvest is automatic now — just
walk.") stays for now — muscle memory takes longer than two weeks to
fade and the redirect costs 7 lines.

adv2 scenario test rewired to drive autoHarvestRoom directly.

internal/plugin/dnd_resource_registry.go

@@ -127,7 +127,7 @@ var zoneResources = map[ZoneID][]ZoneResource{
 		{ID: "hag_hair", Name: "Hag Hair", Action: HarvestCommune, DC: 18, Rarity: RarityRare, Type: "material", SellValue: 220, MaxCharges: 2, ClassRestrict: ClassCleric, RequiresKill: "hag"},
 		{ID: "timelock_amber", Name: "Timelock Amber", Action: HarvestForage, DC: 21, Rarity: RarityRare, Type: "material", SellValue: 290, MaxCharges: 2},
 		{ID: "thornmother_thorn", Name: "Thornmother's Thorn", Action: HarvestScavenge, DC: 25, Rarity: RarityVeryRare, Type: "material", SellValue: 1000, MaxCharges: 1, RequiresKill: "thornmother"},
-		// §6.1 fish — fey streams; time-distortion event hook fires in handleHarvestCmd.
+		// §6.1 fish — fey streams; time-distortion event hook fires in the autopilot harvest pass.
 		{ID: "moonlit_minnow", Name: "Moonlit Minnow", Action: HarvestFish, DC: 12, Rarity: RarityCommon, Type: "fish", SellValue: 12, MaxCharges: 2},
 		{ID: "dreaming_pike", Name: "Dreaming Pike", Action: HarvestFish, DC: 16, Rarity: RarityUncommon, Type: "fish", SellValue: 55, MaxCharges: 2},
 		{ID: "timeworn_koi", Name: "Timeworn Koi", Action: HarvestFish, DC: 21, Rarity: RarityRare, Type: "fish", SellValue: 280, MaxCharges: 2},