feat(ghostwriter): branching conversations with edit and per-message regenerate (#290)

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.

shared/src/types/chat.ts

@@ -29,6 +29,7 @@ export interface JobChatThread {
   createdAt: string;
   updatedAt: string;
   lastMessageAt: string | null;
+  activeRootMessageId: string | null;
 }
 
 export interface JobChatMessage {
@@ -42,10 +43,21 @@ export interface JobChatMessage {
   tokensOut: number | null;
   version: number;
   replacesMessageId: string | null;
+  parentMessageId: string | null;
+  activeChildId: string | null;
   createdAt: string;
   updatedAt: string;
 }
 
+export interface BranchInfo {
+  /** The message ID this branch info belongs to (the currently active sibling). */
+  messageId: string;
+  /** Ordered sibling IDs at this branch point (by createdAt). */
+  siblingIds: string[];
+  /** 0-based index of the active sibling within siblingIds. */
+  activeIndex: number;
+}
+
 export interface JobChatRun {
   id: string;
   threadId: string;