fix: keep background process notifications out of model input

[acc001k]

Summary

• keep CLI background process completion notifications on a display-only path instead of feeding them back as pending user input
• deliver gateway background process notifications directly to the original chat target instead of re-entering the message handler as internal user input
• contain shell option side effects from local background commands by running user commands inside a subshell wrapper

Test Plan

• python -m py_compile cli.py gateway/run.py tools/process_registry.py hermes_cli/kanban_db.py
• git diff --check origin/main...HEAD
• python -m pytest -o addopts= -n 4 --ignore=tests/integration --ignore=tests/e2e -m 'not integration' tests/cli/test_cli_background_process_notifications.py tests/gateway/test_background_process_notifications.py tests/tools/test_notify_on_complete.py

Targeted pytest result: 57 passed.