feat(tools): enhance ExecTool with enable flag and custom deny_patterns

- Add `enable` flag to `ExecToolConfig` to conditionally register the tool.
- Add `deny_patterns` to allow users to override the default command blacklist.
- Remove `allow_patterns` (whitelist) to maintain tool flexibility.
- Fix initialization logic to properly handle empty list (`[]`), allowing users to completely clear the default blacklist.

nanobot/config/schema.py

@@ -305,9 +305,10 @@ class WebToolsConfig(Base):
 class ExecToolConfig(Base):
     """Shell exec tool configuration."""
 
+    enable: bool = True
     timeout: int = 60
     path_append: str = ""
-
+    deny_patterns: list[str] | None = None
 
 class MCPServerConfig(Base):
     """MCP server connection configuration (stdio or HTTP)."""