[FEATURE] Allow custom skills to run as /btw-style side-channel commands

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• Searched: "btw side channel skill", "side conversation custom", "btw tool access" — no duplicates found
• [x] This is a single feature request (not multiple features)

Problem Statement

The /btw command provides a non-interrupting interaction pattern — you can type it mid-task, get an answer in a lightweight side channel, and return to the main thread without any context pollution or interruption. However, /btw is chat-only with no tool access, and there's no way for custom skills to opt into this side-channel execution mode.

This means lightweight operations that should be quick and non-disruptive — like appending a note to a file, bookmarking a reference, or logging an observation — require interrupting the main task thread.

Proposed Solution

Allow custom skills to opt into /btw-style side-channel execution via a SKILL.md frontmatter option, e.g.:

---
name: quick-note
description: Append a note to a scratchpad
side-channel: true
---

When invoked, the skill would run in the same non-interrupting side context as /btw, but with access to a limited set of tools (at minimum file read/write/append). The main thread continues uninterrupted and no tokens are appended to the main session record.

Alternative Solutions

• Using /btw and mentally noting ideas: works for recall but can't persist to disk since /btw has no tool access
• Waiting for a natural pause to run the skill normally: briefly interrupts and risks forgetting the idea
• Typing raw notes in chat: pollutes main context and still can't write to files

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

1. Claude is mid-way through a multi-file refactor
2. While reviewing its output, I think of something unrelated I want to capture
3. I type /quick-note "look into switching to the new token format"
4. The note is appended to a file in the side channel — no interruption to the main task
5. The refactor continues exactly where it left off

Additional Context

The /btw side-channel architecture already exists — this is a request to open it up to custom skills with minimal tool access.