feat: add hooks system with on_exit interceptor for interactive mode

[chaliy]

Summary

• Add hooks module with HookAction<T>, Interceptor<T> types and Hooks registry
• Implement on_exit hook fired by the interpreter when ControlFlow::Exit is processed
• Wire interactive REPL to use the hook for exit / exit N session termination
• Hooks registered via BashBuilder::on_exit(), frozen at build time — no mutex

Design

All hooks are interceptors: they receive owned data and return HookAction::Continue(data) (possibly modified) or HookAction::Cancel(reason). Sync callbacks only — async consumers bridge via channels/atomics.

Only on_exit is wired up. Other hooks (before_exec, after_tool, before_http, etc.) will follow in #1235.

Test plan

• on_exit_hook_fires — hook called on bare exit
• on_exit_hook_carries_code — exit 42 passes code through
• on_exit_hook_fires_in_command_list — echo bye; exit 1 works through pipeline
• on_exit_hook_not_fired_for_normal_commands — echo hello does not trigger hook
• on_exit_hook_code_truncated_to_byte — exit 256 truncates to 0
• Builder doctest for BashBuilder::on_exit()
• All 91 CLI tests pass
• clippy clean, fmt clean