Add session-scoped workspace switching

[benjaminshafii]

Summary

• surface a session-specific workspace list in the session sidebar with add/switch controls
• persist workspace selections per session and auto-pin the active workspace
• wire session view to pass workspace switching state into the sidebar

Testing

• bun run test:session-switch
• bun run test:sessions

[github-actions]

The following comment was made by an LLM, it may be inaccurate: