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/ipc.ts

@@ -58,6 +58,11 @@ export interface DuplicateSessionInput {
   sessionId: string;
 }
 
+export interface BranchSessionInput {
+  sessionId: string;
+  messageId: string;
+}
+
 export interface RenameSessionInput {
   sessionId: string;
   title: string;
@@ -174,6 +179,7 @@ export interface ElectronApi {
   updateSessionApprovalSettings(input: UpdateSessionApprovalSettingsInput): Promise<WorkspaceState>;
   createSession(input: CreateSessionInput): Promise<WorkspaceState>;
   duplicateSession(input: DuplicateSessionInput): Promise<WorkspaceState>;
+  branchSession(input: BranchSessionInput): Promise<WorkspaceState>;
   renameSession(input: RenameSessionInput): Promise<WorkspaceState>;
   setSessionPinned(input: SetSessionPinnedInput): Promise<WorkspaceState>;
   setSessionArchived(input: SetSessionArchivedInput): Promise<WorkspaceState>;