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/pagination.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from typing import TypeVar
+
+from sqlalchemy.sql import Select
+
+SelectT = TypeVar("SelectT", bound=Select)
+
+
+@dataclass(frozen=True)
+class PageParams:
+    limit: int = 100
+    offset: int = 0
+
+    def __post_init__(self) -> None:
+        if self.limit < 1 or self.limit > 1000:
+            raise ValueError("limit must be between 1 and 1000")
+        if self.offset < 0:
+            raise ValueError("offset must be >= 0")
+
+
+def apply_pagination(query: SelectT, page: PageParams) -> SelectT:
+    return query.offset(page.offset).limit(page.limit)