A Gateway timeout or closed connection now fails the command with an
actionable stderr hint instead of silently re-running the whole turn
embedded under a fresh gateway-fallback-* session. The silent fallback
could double-execute side effects (the Gateway may still finish an
accepted turn), returned context-free answers to --session-key callers,
and ran with the CLI host's local config. --local remains the only
embedded execution path.
Also deletes the resultMetaOverrides plumbing (the fallback was its only
writer) and the fallback marker fields added in #111645.
c5254f13ee