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/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),