fix: default exec security to allowlist

docs/tools/exec.md

@@ -39,7 +39,7 @@ Notes:
 
 - `tools.exec.notifyOnExit` (default: true): when true, backgrounded exec sessions enqueue a system event and request a heartbeat on exit.
 - `tools.exec.host` (default: `sandbox`)
-- `tools.exec.security` (default: `deny`)
+- `tools.exec.security` (default: `deny` for sandbox, `allowlist` for gateway + node when unset)
 - `tools.exec.ask` (default: `on-miss`)
 - `tools.exec.node` (default: unset)
 - `tools.exec.pathPrepend`: list of directories to prepend to `PATH` for exec runs.