Conversation Branching and Side-by-Side Branch Compare

Summary

Allow users to branch from any earlier message in a conversation, continue the branch as a separate thread, and compare outcomes side-by-side.

Problem

Current conversations are linear. If a user wants to explore an alternative prompt/path, they must overwrite context in the same thread or manually create a new conversation, losing traceability.

Proposed Solution

Add first-class branching to conversation history so users can fork from any message and explore alternatives without losing the original path.

Backend

• Extend conversation/message metadata to support branching:
  • parentConversationId (optional)
  • branchedFromMessageId (optional)
  • branchLabel (optional)
• Add endpoints:
  • POST /api/conversations/:id/branch (fork from a message)
  • GET /api/conversations/:id/branches (list child branches)
  • GET /api/conversations/:id/compare/:branchId (return normalized compare payload)
• Preserve immutable source thread messages; branch starts with copied context up to fork point.

Frontend

• Add “Branch from here” action on user/assistant messages.
• Add branch switcher in sidebar (tree or grouped list).
• Add branch compare view:
  • message count, latest response, citation count, last updated
  • optional side-by-side transcript panel

Docs

• Update openapi.yaml for branch and compare endpoints.
• Document conversation branching behavior in README.

Acceptance Criteria

• User can fork from any message in an authenticated conversation.
• New branch keeps context up to fork point and diverges independently afterward.
• Original conversation remains unchanged.
• Branches are discoverable and selectable in UI.
• Compare endpoint/view shows key differences between base and branch.
• Tests cover branching logic, auth boundaries, and compare payload correctness.

Out of Scope

• Real-time multi-user collaborative editing.
• Merge-back of branch content into base conversation.

Suggested Tasks

• Add branch metadata to conversation model.
• Implement branch create/list/compare endpoints.
• Add UI branch action + branch navigation.
• Add compare view and summary metrics.
• Add integration tests + docs updates.