Feature request: prompt history sidebar with reusable snippets and keybindings

## Summary

It would be useful to have a prompt/message history view that makes previously sent user messages easy to navigate, reuse, and turn into lightweight commands.

## Problem

In longer Mux sessions, useful prompts are often buried in the timeline. I may want to:

- jump back to a previous user message or its surrounding conversation context
- reuse or slightly edit a prompt I already sent
- keep a few frequently used prompts as reusable commands
- trigger those commands quickly from the keyboard

Right now, this is doable manually by scrolling/searching, but it is not a first-class workflow.

## Proposed UX

A possible design:

- Add a right-side prompt history panel showing user-sent messages from oldest to newest.
- Each entry can be clicked to jump/scroll to the original message in the conversation.
- Each entry has actions such as copy, insert into composer, edit before sending, and save as snippet.
- Saved snippets/macros can optionally be bound to a keybinding or slash command.
- The panel could support filtering by text, agent/session, or saved-only entries.

## MVP scope

A small first version could be:

- read the existing session chat history
- render user messages in a sidebar
- click an item to focus the corresponding message
- provide copy/insert actions

A later version could add persistent snippets, slash command integration, and configurable keybindings.

## Why this fits Mux

Mux already has a keyboard-driven workflow, slash suggestions, command palette patterns, and persistent session history. This feature would make prompt reuse and navigation feel native instead of requiring manual scrolling or copy/paste.

## Contribution offer

If this direction fits the project, I would be happy to help implement it. I can start with the MVP version first and keep snippets/keybindings as follow-up work if maintainers prefer a smaller PR.