fix(rules): scale the bull rush push distance with the margin

PF1e (SRD Bull Rush): "your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent's CMD you can push the target back an additional 5 feet." _bull_rush_push (and the explicit-direction branch) now walk 1 + margin//5 cells, stopping at the first obstacle; a natural-20 auto-success below CMD floors the margin at 0 (base push only). README updated.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_creature_sizes.py

@@ -534,7 +534,7 @@ class TestBullRushLarge:
         t_state = make_state(target, "e", (3, 3))
         engine = make_engine([a_state, t_state])
         # Push direction: away from attacker (3,1) -> (3,3) means push right
-        result = engine._bull_rush_push(a_state, t_state)
+        result = engine._bull_rush_push(a_state, t_state, 1)
         if result:
             # Target moved 1 cell right
             assert t_state.pos == (3, 4)