feat: supabase stack seeds a requires on the substrate at create

A stack's substrate dependency now becomes a real encoded `requires` at
`castle program create` time (STACK_REQUIRES), so the relationship graph shows it.
This keeps `stack` uncoupled from the runtime model — the stack declares the edge
once at creation; the graph only ever reads the encoded `requires`, never the stack.

cli/src/castle_cli/manifest.py

@@ -21,6 +21,7 @@ from castle_core.manifest import (  # noqa: F401 — explicit re-exports for typ
     Reach,
     ReadinessHttpGet,
     RemoteDeployment,
+    Requirement,
     RestartPolicy,
     RunCommand,
     RunCompose,