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/shared/contracts/channels.ts

@@ -31,6 +31,7 @@ export const ipcChannels = {
   updateSessionApprovalSettings: 'sessions:update-approval-settings',
   createSession: 'sessions:create',
   duplicateSession: 'sessions:duplicate',
+  branchSession: 'sessions:branch',
   renameSession: 'sessions:rename',
   setSessionPinned: 'sessions:set-pinned',
   setSessionArchived: 'sessions:set-archived',