Introduce `ToolApprovalFn` callback in `SessionConfig` to gate tool
execution by permission level. Create `agent-cli` crate as a thin CLI
binary wrapping `Session` with provider/model resolution, permission
model (read-only/read-write/full), interactive approval prompts,
real-time event rendering, debug middleware, and SIGINT handling.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>