feat(gateway): configurable background process watcher notifications

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.

cli-config.yaml.example

@@ -655,6 +655,15 @@ display:
   # Toggle at runtime with /verbose in the CLI
   tool_progress: all
 
+  # Background process notifications (gateway/messaging only).
+  # Controls how chatty the process watcher is when you use
+  # terminal(background=true, check_interval=...) from Telegram/Discord/etc.
+  #   off:     No watcher messages at all
+  #   result:  Only the final completion message
+  #   error:   Only the final message when exit code != 0
+  #   all:     Running output updates + final message (default)
+  background_process_notifications: all
+
   # Play terminal bell when agent finishes a response.
   # Useful for long-running tasks — your terminal will ding when the agent is done.
   # Works over SSH. Most terminals can be configured to flash the taskbar or play a sound.