docs: expand package comment in internal/runtime (gt-va49s)

Expanded the package comment to better describe what the runtime package
handles: LLM backend integration, session ID detection, hook installation,
startup fallbacks, and role-specific behavior configuration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: furiosa

internal/runtime/runtime.go

@@ -1,4 +1,8 @@
-// Package runtime provides helpers for runtime-specific integration.
+// Package runtime provides helpers for runtime-specific integration across
+// different LLM agent backends (Claude, OpenCode). It handles session ID
+// detection, hook installation, startup fallback commands for environments
+// without native hooks, and role-specific behavior configuration for
+// autonomous vs human-guided agent roles.
 package runtime
 
 import (