Add hub-core package, docs, and State Hub integration scaffold

Extract the first reusable slice (models, schemas, routers, MCP, migrations)
from state-hub with INTENT/SCOPE, agent instructions, workplan, and aligned
inter_hub capability registry index.

hub_core/utils/paths.py

@@ -0,0 +1,13 @@
+import socket
+from typing import Protocol
+
+
+class RepoPathLike(Protocol):
+    local_path: str | None
+    host_paths: dict
+
+
+def resolve_repo_path(repo: RepoPathLike, host: str | None = None) -> str | None:
+    selected_host = host or socket.gethostname()
+    host_paths = repo.host_paths or {}
+    return host_paths.get(selected_host) or repo.local_path