Auto-Reply Toggle for Recent Agent in Threads

## Summary
Add an optional toggle to automatically direct replies to the last invoked agent in a thread when that agent has been mentioned in the thread or the originating message.

## Problem
In multi-agent conversations, manually specifying which agent to reply to creates friction. When working with an agent across several messages, each reply requires explicitly re-mentioning the agent or re-selecting the target.

## Proposed Solution
Implement a global toggle (opt-in) that:
- Detects when an agent has been mentioned in the current thread or the thread-starting message
- Automatically routes new replies to that last invoked agent
- Remains active until the user disables it

This mirrors the mental model from Slack threads where conversation flows naturally within a thread without re-tagging participants on every message.

## Scope
- Toggle state is thread-local
- Off by default
- Clears when switching channels or threads

Start by tracing how thread-local state, channel or thread switching, agent mentions, and reply routing are represented; the issue does not name specific files or tests. Define the toggle's state transitions and verify that replies route to the last mentioned agent only when enabled, remain off by default, and clear when switching channels or threads.

Written by the indexing model from the issue text.