fix: make module-mode CI green (no local-only replace directives)

The feature branch pointed go.mod at local sibling checkouts (../harrier,
../shrike, ../kestrel, ../merlin, ../swift, ../graycode-router) that do not
exist as reachable GitHub repos, so GOWORK=off CI could not resolve them. Restore
module-mode compatibility so engines resolve from the committed modproxy (as
main does):

- Remove local replace directives from go.mod/go.sum (regenerate clean in module
  mode; no new deps required)
- Gate internal/provider/gateway/media_backends.go behind //go:build media_engine
  (it uses graycode-router GenerateImage/Transcribe facade not in published v0.0.1)
  and add a no-op stub for normal builds
- Revert engine-version-skewed test expectations (smart_reader, consensus) to the
  published-engine values used by CI
- Fix stale golden help file (swift command description)

Author: Patel230