feat(channels): Allow toggling channels on/off within a running session

Problem

The --channels flag is currently a startup-only parameter. Once a session starts with --channels, there is no way to pause/resume channel connectivity mid-session.

This creates an all-or-nothing choice: either run with channels (losing AskUserQuestion, plan-mode tools) or run without channels (losing remote message handling).

Use Case

Users who work with both terminal and remote channels (Discord, Telegram) often need to:

1. Receive channel messages and respond → channels ON
2. Run interactive workflows (skills with AskUserQuestion, plan approvals) → channels OFF
3. Resume channel monitoring → channels ON

Currently this requires exiting and restarting the session, losing context.

Proposed Solution

Add a runtime toggle, consistent with existing patterns:

/channels pause    # temporarily disable channel message handling + re-enable AskUserQuestion
/channels resume   # re-enable channel handling + disable blocking tools
/channels status   # show current state

This mirrors the existing /mcp enable/disable and /plugin enable/disable patterns.

Why This Matters

Combined with #38498 (channel relay for AskUserQuestion), this provides two complementary solutions:

• Short term: /channels pause as a workaround when interactive tools are needed
• Long term: #38498 channel relay eliminates the need to pause at all

Related

• #38498 — Extend channel relay to AskUserQuestion and all modal dialogs