docs: codify entity systems pattern and record combat extensions

AGENTS.md

@@ -86,6 +86,25 @@ Current resource gathering should use finite `ResourceNode` instances:
 - food: berries, animal camps, village stock, future farms/crops;
 - wood: trees, wood piles, future forestry contexts.
 
+## Entity systems pattern
+
+Build every resource, enemy, and animal as a reusable system with shared root
+behaviour, not as a one-off object. Add a new type through data/definitions
+plus a small behaviour or visual hook, reusing the root for movement,
+presentation, serialization, and lifecycle:
+
+- resources: `ResourceNode` + `ResourceStateRecord` (amount, yield, regrowth);
+- creatures: `CreatureVisual` shared movement root (navigate to the
+  authoritative simulation position, report position, death), with per-type
+  visuals built from definitions;
+- enemies: `EnemyDefinition` + `SimulationEnemies` registry driving combatant
+  spawns and hostile visuals;
+- animals: `AnimalNode` + `AnimalStateRecord` following the same
+  follow-the-authoritative-position contract.
+
+A new berry, tree, bear, bandit, boar, or goat should be mostly a definition
+plus a bounded hook — never a new movement/combat/save system.
+
 Resource additions should preserve:
 
 - stable unique IDs;