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/preload/index.ts

@@ -47,6 +47,7 @@ const api: ElectronApi = {
     ipcRenderer.invoke(ipcChannels.updateSessionApprovalSettings, input),
   createSession: (input) => ipcRenderer.invoke(ipcChannels.createSession, input),
   duplicateSession: (input) => ipcRenderer.invoke(ipcChannels.duplicateSession, input),
+  branchSession: (input) => ipcRenderer.invoke(ipcChannels.branchSession, input),
   renameSession: (input) => ipcRenderer.invoke(ipcChannels.renameSession, input),
   setSessionPinned: (input) => ipcRenderer.invoke(ipcChannels.setSessionPinned, input),
   setSessionArchived: (input) => ipcRenderer.invoke(ipcChannels.setSessionArchived, input),