perf(tools): cache ToolRegistry.get_definitions() between mutations

get_definitions() sorts tools on every LLM iteration for prompt cache
stability.  Cache the sorted result and invalidate on register/unregister
so the sort only runs when the tool set actually changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nanobot/agent/tools/registry.py

@@ -14,14 +14,17 @@ class ToolRegistry:
 
     def __init__(self):
         self._tools: dict[str, Tool] = {}
+        self._cached_definitions: list[dict[str, Any]] | None = None
 
     def register(self, tool: Tool) -> None:
         """Register a tool."""
         self._tools[tool.name] = tool
+        self._cached_definitions = None
 
     def unregister(self, name: str) -> None:
         """Unregister a tool by name."""
         self._tools.pop(name, None)
+        self._cached_definitions = None
 
     def get(self, name: str) -> Tool | None:
         """Get a tool by name."""
@@ -46,8 +49,12 @@ class ToolRegistry:
         """Get tool definitions with stable ordering for cache-friendly prompts.
 
         Built-in tools are sorted first as a stable prefix, then MCP tools are
-        sorted and appended.
+        sorted and appended.  The result is cached until the next
+        register/unregister call.
         """
+        if self._cached_definitions is not None:
+            return self._cached_definitions
+
         definitions = [tool.to_schema() for tool in self._tools.values()]
         builtins: list[dict[str, Any]] = []
         mcp_tools: list[dict[str, Any]] = []
@@ -60,7 +67,8 @@ class ToolRegistry:
 
         builtins.sort(key=self._schema_name)
         mcp_tools.sort(key=self._schema_name)
-        return builtins + mcp_tools
+        self._cached_definitions = builtins + mcp_tools
+        return self._cached_definitions
 
     def prepare_call(
         self,