Add cleanup command for session plugin

[kuiwang02]

Problem
The /save-session command (#11) creates session files in the repository root directory. Over time, these files can accumulate and create clutter:

• session-2025-10-16-feature-work.md
• session-2025-10-17-bug-fixes.md
• session-2025-10-18-pr-review.md
• ...

Users are left to manually identify and delete old session files they no longer need. This can lead to:

• Repository root directory becoming cluttered with old session files
• Difficulty identifying which sessions are still relevant
• Risk of accidentally deleting important sessions
• Manual cleanup burden (rm session-*.md)

Solution
Add a /cleanup-sessions command (or similar) to help users manage their session files. This should:

1. List existing sessions with metadata (date, size, age)
2. Provide filtering options such as:
   • Age-based: sessions older than N days
   • Pattern-based: sessions matching a description pattern
   • Interactive selection: let users choose which to delete
3. Safe deletion with confirmation to prevent accidental data loss
4. Optional automation: e.g., /save-session --cleanup-old to clean up while saving

Design Questions to Consider

• Should this be a separate /cleanup-sessions command or an option to /save-session?
• What default criteria should be used for cleanup? (age-based, count-based, interactive?)
• Should deleted files be moved to trash/backup location instead of permanent deletion?
• Should cleanup work across all repositories or only the current one?

Context
This was suggested by @brandisher in PR #11 review: #11 (review)

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