feat(ui): cascade undo and redo to subagents

packages/ui/src/sync/DOCUMENTATION.md

@@ -268,6 +268,7 @@ Rules:
 6. After session creation, the directory returned by the server is authoritative over the requested draft directory. The server may canonicalize a worktree path, and the first prompt must use the same directory identity as the created session.
 7. Regular new-chat drafts that inherit the persisted current/last directory must not create a session against a confirmed-missing path. Fall back to the active project only when OpenCode reports the directory missing; keep explicit worktree targets, in-flight worktree creation, and unknown/offline probes unchanged, and do not persist the fallback until session creation succeeds. A concurrent draft rewrite to that same active-project fallback must not abort session creation.
 8. A prompt send that fails **after** the request left the client is ambiguous, never a definite failure: the server may already be answering it. Transports tag those errors (`markAmbiguousTransportFailure` in `@/lib/relay/transport-error`; the relay tunnel tags every stream that dies with a request in flight), and `isAmbiguousSendFailure` reads the tag before falling back to status/text heuristics. An ambiguous failure waits for the connection to return, refetches recent messages, and confirms the optimistic message in place instead of rolling it back — rolling it back lets the message queue re-send a prompt the engine is already running, producing two independent AI responses for one user message.
+9. Revert and unrevert cascade through known descendant sessions before mutating the parent. Revert uses the first descendant user message at or after the parent's target timestamp, including equal timestamps because message IDs do not define chronology. A descendant failure is logged and does not block its siblings or the parent. The parent runs last so its shared-directory file snapshot remains authoritative.
 
 Examples of global-store updates performed in `session-actions.ts`: