Add display.background_process_notifications config option to control
how chatty the gateway process watcher is when using
terminal(background=true, check_interval=...) from messaging platforms.
Modes:
- all: running-output updates + final message (default, current behavior)
- result: only the final completion message
- error: only the final message when exit code != 0
- off: no watcher messages at all
Also supports HERMES_BACKGROUND_NOTIFICATIONS env var override.
Includes 12 tests (5 config loading + 7 watcher behavior).
Inspired by @PeterFile's PR #593. Closes#592.
