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/schemas/domain.py

@@ -0,0 +1,49 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class DomainCreate(BaseModel):
+    slug: str
+    name: str
+    description: str | None = None
+
+
+class DomainUpdate(BaseModel):
+    name: str | None = None
+    description: str | None = None
+    status: str | None = None
+
+
+class DomainRename(BaseModel):
+    new_slug: str
+    new_name: str
+
+
+class RepoStub(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    slug: str
+    name: str
+    local_path: str | None = None
+    remote_url: str | None = None
+    status: str
+
+
+class DomainRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    slug: str
+    name: str
+    description: str | None = None
+    status: str
+    created_at: datetime
+    updated_at: datetime
+
+
+class DomainDetail(DomainRead):
+    repos: list[RepoStub] = Field(default_factory=list)
+    extension_counts: dict[str, int] = Field(default_factory=dict)