add reachability pre-checks to plant/sleep/chop/mine

Provider audit found 6 WorkProviders missing reachability gates before
returning jobs. Without them, pawns can be offered doomed walk-jobs
(target boxed in), JobRunner cancels each tick, Decision re-offers
same job → 20Hz busy-spin starves lower-priority work.

Fixed 4 here (mechanical pattern):
- PlantProvider._find_harvest: walkable-target check (mirrors _find_sow)
- SleepProvider: walkable bed-tile check
- ChopProvider: adjacent-walkable for impassable tree
- MineProvider: adjacent-walkable for impassable rock

Cooking/Crafting reachability changes (in the same audit's
recommendation) were attempted but caused intermittent null returns
that regressed cooking rate. Reverted those — they need more careful
work that doesn't break the existing flow. Filed separately.

Future cleanup: _find_adjacent_walkable duplicated across
ConstructionProvider, ChopProvider — extract to a base/util.

MCP-verified after revert: 2 meals + 1 bread + 2 grain in cabin crate
within 3700 ticks at ULTRA. Cooking fires, hauling fires, all
production paths operational.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scenes/ai/plant_provider.gd

@@ -63,6 +63,10 @@ func _find_harvest(pawn) -> Job:
 			continue
 		if Job.is_target_taken_by_other(crop, pawn):
 			continue
+		# Reachability pre-check — mirrors _find_sow / HaulingProvider pattern.
+		if pawn.tile != crop.tile and World.pathfinder != null:
+			if World.pathfinder.find_path(pawn.tile, crop.tile).is_empty():
+				continue
 		var d: int = abs(crop.tile.x - pawn.tile.x) + abs(crop.tile.y - pawn.tile.y)
 		if d < best_dist:
 			best_dist = d