refactor: /btw is now an alias for /background (#16053)

The ephemeral no-tools side-question variant of /btw confused users who
expected 'by-the-way' to mean 'run this off to the side with tools' —
they'd type /btw and get a toolless agent that couldn't do the work.
/bg worked because it was /background with full tools.

Collapse the two: /btw and /bg both alias to /background. One command,
one behavior, no more gotchas about which variant has tools.

Removed:
- _handle_btw_command in cli.py and gateway/run.py
- _run_btw_task + _active_btw_tasks state in gateway/run.py
- prompt.btw JSON-RPC method + btw.complete event in tui_gateway
- BtwStartResponse type + btw.complete case in ui-tui
- Standalone /btw slash tree registration in Discord
- Standalone btw CommandDef in hermes_cli/commands.py

Updated:
- background CommandDef aliases: (bg,) -> (bg, btw)
- TUI session.ts: local btw handler merged into background
- Docs and tips updated to describe /btw as a /background alias

ui-tui/src/app/createGatewayEventHandler.ts

@@ -431,12 +431,6 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
 
         return
 
-      case 'btw.complete':
-        dropBgTask('btw:x')
-        sys(`[btw] ${ev.payload.text}`)
-
-        return
-
       case 'subagent.spawn_requested':
         // Child built but not yet running (waiting on ThreadPoolExecutor slot).
         // Preserve completed state if a later event races in before this one.