feat(rules): conform bulls strength, bears endurance and shield to the SRD

Bull's Strength now grants the SRD +4 enhancement bonus to Strength as
+2 melee attack, +2 melee damage and +2 CMB (three modifiers instead of
the previous +1/+1 approximation). Shield now uses a real shield bonus
type (+4 AC) instead of approximating it as deflection: the engine's
BonusType literal and stacking docs gain the shield type, which stays
out of the CMD bonus list (SRD Combat Maneuver Defense). Bear's
Endurance keeps its +2 Fortitude / +2 hp per HD and its description now
cites the SRD and flags the hp modifier as informational. Golden
transcripts updated: the buff log reports 3 mods and Tammara's pistol
attacks gain the extra +1 enhancement (d20=10+9-6=15).

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

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

data/spells/shield.json

@@ -10,10 +10,10 @@
     {
       "type": "buff",
       "modifiers": [
-        { "target": "ac", "value": 4, "bonus_type": "deflection" }
+        { "target": "ac", "value": 4, "bonus_type": "shield" }
       ],
       "duration_rounds": 10
     }
   ],
-  "description": "An invisible shield of force hovers in front of you, granting a +4 shield bonus to AC for 1 min./level (modeled as 10 rounds; the shield bonus type is approximated as deflection since the engine has no shield bonus type). Negating magic missile and the ranged-attack deflection are not modeled."
+  "description": "An invisible shield of force hovers in front of you, granting a +4 shield bonus to AC for 1 min./level (modeled as 10 rounds). Negating magic missile and the ranged-attack deflection are not modeled."
 }