docs: add package docs to runner and testrun pipeline

internal/runner/runner.go

@@ -1,3 +1,4 @@
+// Package runner drives the observe-decide-act loop that steps a spec against a device.
 package runner
 
 import (

internal/testrun/testrun.go

@@ -1,3 +1,4 @@
+// Package testrun wires together the device, bundler, runner, and verifier into a single test pipeline.
 package testrun
 
 import (