Stop native Postgres between runtime flows

[AntoineToussaint]

Stop now terminates the Nix-hosted Postgres process while preserving its persistent data directory. This releases the assigned port and data lock between validation and CI phases, so the next Runtime.Init can reuse the cluster instead of colliding with an orphaned postmaster.

Docker retains its existing persistent-container behavior.

Regression evidence:

• codefly test source --target ./... --timeout 10m: 173/173
• codefly agent build: packaged and installed postgres 0.0.117
• codefly agent ci: 6/6, including the repeated generated-service lint/compile/test conformance sequence that previously failed with a live postmaster lock/port collision