feat: add /btw background session command

[brendandebeasi]

Issue for this PR

Closes #16992
Related: #17189 (original community implementation by @kjellknapen)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a /btw slash command that lets users fire off lightweight background questions or append todos without interrupting the active session.

This is an alternative implementation to #17189, built on top of the existing session infrastructure with a different approach to background session management and message reporting.

Key changes:

• /btw ask <question> — spawns a background session that inherits parent context, runs the question, and reports the result back as a system message (without polluting provider message history)
• /btw todo <item> — appends a todo to the current session's todo list
• /btw status — shows status of background sessions
• Background session completion is reported back into the parent session cleanly
• Adds regression tests for /btw behavior and synthetic assistant message serialization

How did you verify your code works?

• bun test test/session/btw.test.ts test/session/message-v2.test.ts
• bun typecheck
• bun test (full suite)

Screenshots / recordings

N/A — backend/CLI command, no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found a potential duplicate:

PR #17189: feat: Add the /btw command
#17189

This PR appears to be addressing the same feature as the current PR #17198. Both are implementing the /btw background session command. You should verify if one supersedes the other or if they're approaching the same feature differently.