[FEATURE] Add ability to resume from other session despite current ongoing session

## Feature Description

Right now, you can resume from a previous session when Claude Code hasn't yet been toggled on yet. However, if I want to potentially switch to another Claude Code session while you've already toggled it, then you need to 1) go to the current Claude Code session, 2) terminate the session, and 3) trigger `:ClaudeCode --resume`.

Ideally, we can just trigger something like `:ClaudeCode --resume` and that would just pull up the list of previous Claude Code session.

## Use Case

This would make Claude Code session management much easier.

## Proposed Solution

A nice solution would be to add a more generic `:ClaudeCodeCommand` that allows users to send arbitrary Claude Code commands, e.g. `:ClaudeCodeCommand clear` would run `/clear` within the Claude Code session.

## Alternatives Considered

The alternative is to 1) go to the current Claude Code session, 2) terminate the session, and 3) trigger `:ClaudeCode --resume`.

## Additional Context

Start by tracing the implementation of `:ClaudeCode --resume` and how an active Claude Code session is managed. Compare the current resume flow with the proposed `:ClaudeCodeCommand` behavior, then determine the intended command semantics and session-switching boundaries. Done means a resume command can show previous sessions without manually terminating the current one, with the relevant behavior covered by tests if the project provides them.

索引モデルが issue の本文から書いたものです。