Turn Ghostwriter's flat message list into a tree structure, enabling
Claude.ai/ChatGPT-style branching conversations.
**Data model**: Add `parentMessageId` and `activeChildId` to messages,
`activeRootMessageId` to threads. Migration backfills existing messages
into a linear chain and links regenerated messages as siblings.
**Backend**: Tree-walking queries (getActivePathFromRoot, getAncestorPath,
getSiblingsOf), rewritten history builder that follows the ancestor path,
new editMessage and switchBranch services, regenerate now works on any
assistant message (not just the latest).
**Frontend**: BranchNavigator component (← 2/3 → arrows), inline edit on
user messages, per-message regenerate on assistant messages, regenerate
button removed from composer (now per-message).
**Infra**: Pin Node 22 via Volta to prevent ABI mismatches with
better-sqlite3 across environments.
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