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

web/src/lib/gatewayClient.ts

@@ -32,7 +32,6 @@ export type GatewayEventName =
   | "sudo.request"
   | "secret.request"
   | "background.complete"
-  | "btw.complete"
   | "error"
   | "skin.changed"
   | (string & {});