feat: add backend session branching support

Add the backend contract and session-domain support for
'Branch from here':
- new branchSession IPC method and sessions:branch channel
- branchOrigin metadata on SessionRecord
- session branching helper that truncates the transcript at a
  chosen user message and clears runtime state
- AryxAppService branching flow with scratchpad directory support
- persistence normalization and regression coverage

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/AryxAppService.ts

@@ -62,6 +62,7 @@ import {
 } from '@shared/domain/approval';
 import { isScratchpadProject, type ProjectRecord } from '@shared/domain/project';
 import {
+  branchSessionRecord,
   duplicateSessionRecord,
   querySessions as queryWorkspaceSessions,
   renameSessionRecord,
@@ -703,6 +704,23 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
     return this.persistAndBroadcast(workspace);
   }
 
+  async branchSession(sessionId: string, messageId: string): Promise<WorkspaceState> {
+    const workspace = await this.loadWorkspace();
+    const session = this.requireSession(workspace, sessionId);
+    const pattern = this.requirePattern(workspace, session.patternId);
+    const branch = branchSessionRecord(session, pattern, createId('session'), messageId, nowIso());
+    if (isScratchpadProject(branch.projectId)) {
+      branch.cwd = undefined;
+    }
+
+    await this.ensureScratchpadSessionDirectory(branch);
+    workspace.sessions.unshift(branch);
+    workspace.selectedProjectId = branch.projectId;
+    workspace.selectedPatternId = branch.patternId;
+    workspace.selectedSessionId = branch.id;
+    return this.persistAndBroadcast(workspace);
+  }
+
   async renameSession(sessionId: string, title: string): Promise<WorkspaceState> {
     const workspace = await this.loadWorkspace();
     const session = this.requireSession(workspace, sessionId);

src/main/ipc/registerIpcHandlers.ts

@@ -3,6 +3,7 @@ import type { BrowserWindow } from 'electron';
 
 import { ipcChannels } from '@shared/contracts/channels';
 import type {
+  BranchSessionInput,
   CancelSessionTurnInput,
   CreateSessionInput,
   DismissSessionMcpAuthInput,
@@ -135,6 +136,9 @@ export function registerIpcHandlers(window: BrowserWindow, service: AryxAppServi
   ipcMain.handle(ipcChannels.duplicateSession, (_event, input: DuplicateSessionInput) =>
     service.duplicateSession(input.sessionId),
   );
+  ipcMain.handle(ipcChannels.branchSession, (_event, input: BranchSessionInput) =>
+    service.branchSession(input.sessionId, input.messageId),
+  );
   ipcMain.handle(ipcChannels.renameSession, (_event, input: RenameSessionInput) =>
     service.renameSession(input.sessionId, input.title),
   );

src/main/persistence/workspaceRepository.ts

@@ -6,7 +6,7 @@ import { isScratchpadProject, mergeScratchpadProject } from '@shared/domain/proj
 import { normalizeDiscoveredToolingState } from '@shared/domain/discoveredTooling';
 import { normalizeProjectCustomizationState } from '@shared/domain/projectCustomization';
 import { normalizeSessionRunRecords } from '@shared/domain/runTimeline';
-import type { SessionRecord } from '@shared/domain/session';
+import { normalizeSessionBranchOrigin, type SessionRecord } from '@shared/domain/session';
 import {
   normalizeSessionToolingSelection,
   normalizeWorkspaceSettings,
@@ -81,6 +81,7 @@ export class WorkspaceRepository {
     const sessions = await Promise.all((stored.sessions ?? []).map(async (session): Promise<SessionRecord> => {
       const normalizedSession: SessionRecord = {
         ...session,
+        branchOrigin: normalizeSessionBranchOrigin(session.branchOrigin),
         runs: normalizeSessionRunRecords(session.runs),
         tooling: normalizeSessionToolingSelection(session.tooling),
         approvalSettings: normalizeSessionApprovalSettings(session.approvalSettings),