[FEATURE] Extend /btw to support running shell commands and invoking skills without interrupting active turn

Feature Request

Currently /btw injects a note into the conversation without interrupting Claude's active turn. This is great for adding context mid-task.

The request is to extend /btw so it can also:

1. Run a shell command in the background while Claude is working, and optionally surface the output as a /btw-style note
2. Invoke a skill (e.g. /btw /compact, /btw /create-continuation) without interrupting the current Claude turn

Current Behavior

/btw <message> — injects plain text context into the running turn, no tool/skill invocation.

Shell commands and skills typed during an active turn are either:

• Queued (running after the current turn finishes)
• Or they interrupt Claude

Desired Behavior

/btw !ls -la          # run shell command, surface output as a note
/btw /compact         # trigger skill without interrupting current turn
/btw /create-continuation  # write handoff file while Claude keeps working

The /btw prefix signals "do this in parallel / non-interruptively."

Why This Matters

• Developers often realize mid-task they want to trigger a skill (e.g. start a continuation file, run a linter) without stopping Claude
• The existing /btw mechanism already establishes the non-interrupting pattern — extending it to shell + skills is a natural progression
• Particularly valuable for remote sessions (Telegram/Discord) where interrupting and re-prompting is costly

Related

• #36761 — /add-dir requested to work outside agent loop like /btw
• #36817 — TUI queue management for pending messages