[FEATURE] JetBrains plugin: persistent sidebar listing past sessions (like Cursor)

**Description**

In Cursor, previous chat/agent sessions are always visible in a sidebar list, so switching back to an earlier session is a single click. The Claude Code JetBrains plugin (PhpStorm/IntelliJ) has no equivalent — the only way to get back to a past session is the `/resume` picker (or `claude --resume` from a terminal), which is a modal-style picker you have to invoke, not an always-visible list.

**Request**

Add a persistent sessions panel/tool window to the JetBrains plugin that lists past sessions for the current project (most recent first), so a session can be reopened with one click instead of invoking `/resume` each time.

**Related**

Related to #87231 (unifying session history *across* IDE integrations) but distinct — this is about surfacing the history that already exists *within* the JetBrains plugin as a persistent UI element, independent of whether cross-IDE unification happens.

The issue does not name files or tests, so first locate the JetBrains plugin code and the existing session resume/history path used by `/resume` or `claude --resume`. Read related issue #87231 for context. Done means the plugin has a persistent per-project sessions panel sorted most-recent-first, and a listed session can be reopened with one click.

索引モデルが issue の本文から書いたものです。