Add manual cleanup command for Agents Window worktrees

[meganrogge]

Feature request

Provide an Agents: Clean Up Session Worktrees... command so users can inspect and reclaim worktree storage without waiting for the automatic worktree-limit prompt.

Proposed behavior

• Make the command available in the Agents Window when the experimental worktree cleanup feature is enabled.
• Show every safe cleanup candidate: completed, inactive, non-archived sessions that exclusively own their worktrees.
• Clearly explain why each row is or is not recommended:
  • old, unpinned sessions are recommended and selected by default;
  • recent sessions remain available but are not selected automatically;
  • pinned sessions remain available but are not selected automatically.
• Show measured worktree disk usage when available.
• Allow users to select additional recent or pinned sessions intentionally.
• Keep active, running, archived, and shared-worktree sessions out of the cleanup list.
• Explain when no worktrees are eligible instead of silently doing nothing.

The automatic prompt should continue to appear only at the unique-worktree threshold and only when at least one recommended old session exists.

Related implementation: #338033