> Lifecycle interceptors and tool execution guards

# Agent Hooks System

Hooks (`internal/hooks/types.go`) allow operators to intercept the agent loop at defined lifecycle checkpoints:

## Supported Hook Events

* `session_start` — Fires when a new agent session is established.
* `user_prompt_submit` — **BLOCKING:** Evaluates or mutates prompt input before entering the pipeline.
* `pre_tool_use` — **BLOCKING:** Intercepts tool calls before execution to enforce security policies or human approval.
* `post_tool_use` — Non-blocking post-execution observation and logging.
* `stop` — Fires upon session completion.
* `subagent_start` — **BLOCKING:** Authorizes sub-agent spawning.
* `subagent_stop` — Finalizes sub-agent task completion.