feat(budget): make tool result persistence thresholds configurable

Add BudgetConfig dataclass to centralize and make overridable the
hardcoded constants (50K per-result, 200K per-turn, 2K preview) that
control when tool outputs get persisted to sandbox. Configurable at
the RL environment level via HermesAgentEnvConfig fields, threaded
through HermesAgentLoop to the storage layer.

Resolution: pinned (read_file=inf) > env config overrides > registry
per-tool > default. CLI override: --env.turn_budget_chars 80000

tools/budget_config.py

@@ -0,0 +1,52 @@
+"""Configurable budget constants for tool result persistence.
+
+Overridable at the RL environment level via HermesAgentEnvConfig fields.
+Per-tool resolution: pinned > config overrides > registry > default.
+"""
+
+from dataclasses import dataclass, field
+from typing import Dict
+
+# Tools whose thresholds must never be overridden.
+# read_file=inf prevents infinite persist->read->persist loops.
+PINNED_THRESHOLDS: Dict[str, float] = {
+    "read_file": float("inf"),
+}
+
+# Defaults matching the current hardcoded values in tool_result_storage.py.
+# Kept here as the single source of truth; tool_result_storage.py imports these.
+DEFAULT_RESULT_SIZE_CHARS: int = 50_000
+DEFAULT_TURN_BUDGET_CHARS: int = 200_000
+DEFAULT_PREVIEW_SIZE_CHARS: int = 2_000
+
+
+@dataclass(frozen=True)
+class BudgetConfig:
+    """Immutable budget constants for the 3-layer tool result persistence system.
+
+    Layer 2 (per-result): resolve_threshold(tool_name) -> threshold in chars.
+    Layer 3 (per-turn):   turn_budget -> aggregate char budget across all tool
+                          results in a single assistant turn.
+    Preview:              preview_size -> inline snippet size after persistence.
+    """
+
+    default_result_size: int = DEFAULT_RESULT_SIZE_CHARS
+    turn_budget: int = DEFAULT_TURN_BUDGET_CHARS
+    preview_size: int = DEFAULT_PREVIEW_SIZE_CHARS
+    tool_overrides: Dict[str, int] = field(default_factory=dict)
+
+    def resolve_threshold(self, tool_name: str) -> int | float:
+        """Resolve the persistence threshold for a tool.
+
+        Priority: pinned -> tool_overrides -> registry per-tool -> default.
+        """
+        if tool_name in PINNED_THRESHOLDS:
+            return PINNED_THRESHOLDS[tool_name]
+        if tool_name in self.tool_overrides:
+            return self.tool_overrides[tool_name]
+        from tools.registry import registry
+        return registry.get_max_result_size(tool_name, default=self.default_result_size)
+
+
+# Default config -- matches current hardcoded behavior exactly.
+DEFAULT_BUDGET = BudgetConfig()