feat: organize conversations into folders

[Radiummm]

Summary

• add project-scoped conversation folders to the session sidebar
• support creating, renaming, collapsing, deleting, and drag-and-drop assignment
• preserve folder state in browser storage and aggregate subagent activity on folder headers

Testing

• node --experimental-strip-types --test lib/conversation-folder-state.test.mjs components/SessionSidebar.test.mjs
• npx tsc --noEmit
• npm run lint
• npm test (850 passing)
• manually verified folder creation and rendering against a populated local session list

[agegr]

Thanks for the contribution. After trying it out, I think session organization needs a different interaction design. I plan to implement grouping separately, so I am closing this PR.