Add /list-sessions and /restore-session commands

[chr15m]

This PR introduces session management capabilities to aider, allowing users to view and restore previous chat contexts from the chat history file.

Key Features:

• /list-sessions: Displays a chronological list of previous chat sessions found in the history file, including timestamps and snippets of the first/last user messages for easy identification.
• /restore-session [N]: Restores the chat context (messages) from a specific previous session.
  • Supports positive indices (from /list-sessions output).
  • Supports negative indices (e.g., -1 for the most recent session).
  • Automatically handles session filtering to exclude tool-only notifications and previous restore commands to keep the history clean.
• Enhanced History Tracking:
  • /run command outputs are now stored in the chat history file.
  • Metadata for restored sessions is appended to the history to maintain a clear audit trail.
• Robust Filtering: Improved logic to filter out aider-internal messages and session management commands when parsing the history for restoration.