core/memory: bind sessions to workspace path with SQLite migrate

Add session.workspace column, create/list filters, and ALTER TABLE
migration for existing chat databases.

tests/test_memory/test_store.py

@@ -59,3 +59,63 @@ async def test_list_sessions(store: MemoryStore) -> None:
     ids = {s.sid for s in sessions}
     assert s1.sid in ids
     assert s2.sid in ids
+
+
+async def test_session_workspace_binding(store: MemoryStore, tmp_path: object) -> None:
+    from pathlib import Path
+
+    assert isinstance(tmp_path, Path)
+    a = tmp_path / "proj-a"
+    b = tmp_path / "proj-b"
+    a.mkdir()
+    b.mkdir()
+    sa = await store.create_session(name="a", workspace=a)
+    sb = await store.create_session(name="b", workspace=b)
+    assert sa.workspace == str(a.resolve())
+    listed_a = await store.list_sessions(workspace=a)
+    assert {s.sid for s in listed_a} == {sa.sid}
+    listed_b = await store.list_sessions(workspace=b)
+    assert {s.sid for s in listed_b} == {sb.sid}
+
+
+async def test_workspace_column_migration(tmp_path: object) -> None:
+    """Existing DBs without session.workspace get the column via ALTER."""
+    from pathlib import Path
+
+    from sqlalchemy import text
+    from sqlalchemy.ext.asyncio import create_async_engine
+
+    assert isinstance(tmp_path, Path)
+    db_path = tmp_path / "legacy.db"
+    engine = create_async_engine(f"sqlite+aiosqlite:///{db_path}")
+    async with engine.begin() as conn:
+        _ = await conn.execute(
+            text(
+                "CREATE TABLE user ("
+                "uid INTEGER PRIMARY KEY, name VARCHAR(48) UNIQUE, "
+                "email VARCHAR(255) UNIQUE, password_hash VARCHAR(256), "
+                "created_at DATETIME)"
+            )
+        )
+        _ = await conn.execute(
+            text(
+                "CREATE TABLE session ("
+                "sid INTEGER PRIMARY KEY, uid INTEGER, name VARCHAR(64), "
+                "created_at DATETIME, updated_at DATETIME)"
+            )
+        )
+        _ = await conn.execute(
+            text(
+                "CREATE TABLE message ("
+                "mid INTEGER PRIMARY KEY, sid INTEGER, seq INTEGER, "
+                "data JSON, created_at DATETIME, updated_at DATETIME)"
+            )
+        )
+    await engine.dispose()
+
+    store = await MemoryStore.open(DatabaseConfig(url=str(db_path)))
+    session = await store.create_session(name="migrated", workspace=tmp_path)
+    from plyngent.memory.database.store import normalize_workspace
+
+    assert session.workspace == normalize_workspace(tmp_path)
+    await store.close()