fix(spells): make the bleed spell bleed

PF1e (SRD spells/bleed.md): "Saving Throw Will negates" and the target takes 1 point of damage per round. Two divergences: the JSON declared a Fortitude save, and the bleeding condition never set the engine's bleed_damage field, so the spell was a no-op. Bleed now saves Will; _apply_condition_effect sets bleed_damage = 1 when a spell applies bleeding, and _decrement_durations lands the final tick as the last bleeding condition expires before clearing the field (heal-path clearing already existed in _stop_bleeding). README updated.

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

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

data/spells/bleed.json

@@ -3,7 +3,7 @@
   "level": 0,
   "school": "necromancy",
   "range": "close",
-  "save": "fort",
+  "save": "will",
   "base_dc": 10,
   "cast_time": "standard",
   "effects": [
@@ -14,5 +14,5 @@
       "save_negates": true
     }
   ],
-  "description": "A cantrip that causes a wound to bleed: the target takes 1 point of damage per round until healed. Fortitude negates. Modeled as the bleeding condition for 1 round."
+  "description": "SRD spells/bleed.md: a living creature at -1 or fewer hit points begins dying, taking 1 point of damage per round; Will negates. Modeled as the bleeding condition for 1 round with bleed_damage = 1 (the engine's bleed tick deals it at the start of the target's turn)."
 }