feat: enhance /sessions with message preview and sorted listing

[yuxiaoyang2007-prog]

Summary

Enhances the /sessions command to help users identify and distinguish between sessions:

• Message preview: Shows the last message (user or assistant) for each session, similar to how claude --resume displays session context in the terminal
• Sorted by activity: Sessions are sorted by most recently active first (updatedAt desc) instead of insertion order
• Current session marker: The active session is marked with a ◀ indicator
• Increased limit: Shows up to 15 sessions (was 10)

Before

Sessions:

abc12345... [active] ~/projects/foo
def67890... [inactive] ~/projects/bar

After

Sessions:

1. abc12345 ◀
    💬 那 iPhone 上试试？打开 Tailscale app → Exit Node…
2. def67890
    💬 这个库不是我自己的github库，是别人的对吧

Implementation details

• Reads the tail (last 64KB) of each session's JSONL file for performance
• Strips markdown formatting (bold, headers, code, lists, links) and collapses to single-line preview
• Filters out system/meta messages (commands, timestamps, system reminders)
• Works cross-platform (uses os.homedir() and path.join())

Test plan

• Verify /sessions shows message previews on Feishu
• Verify current session is marked with ◀
• Verify sessions are sorted by most recent first
• Verify on Telegram and Discord adapters
• Verify performance with many sessions (reads only file tail)

🤖 Generated with Claude Code