feat(cmd): cell and management-plane wiring with feature gates

Wire the manager to run in either cell or management-plane mode, gating
the federator, projector, and per-cell controllers behind feature flags.
Add the feature-gate registry and extend configuration to carry the
downstream kubeconfig and discovery settings each mode needs.

Single-mode project resolution (decoding edge namespace labels into
project identity) lives in the controller package as
NewSingleModeProjectID/NewSingleModeProjectNamespace constructors;
main.go keeps only the wiring.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells