feat: steer command — mid-session context injection #66

Add support for a steer command that lets users inject guidance into the active context mid-session without interrupting the conversation flow. On the Ragger side: store the steered content as a tagged chunk, surface it as a high-priority layer in the recipe. Needs investigation of available hooks in Claude Code (PreToolUse/PostToolUse/Stop hooks in ~/.claude/hooks/), Hermes (on_turn_start, gateway builtin_hooks/), and other supported agents.