chore: officially migrate to submodule (#4502)

* remove apps/core and apps/fern

* fix precommit

* add submodule updates in workflows

* submodule

* remove core tests

* update core revision

* Add submodules: true to all GitHub workflows

- Ensure all workflows can access git submodules
- Add submodules support to deployment, test, and CI workflows
- Fix YAML syntax issues in workflow files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* remove core-lint

* upgrade core with latest main of oss

---------

Co-authored-by: Claude <noreply@anthropic.com>

alembic/versions/220856bbf43b_add_read_only_column.py

@@ -1,44 +0,0 @@
-"""add read-only column
-
-Revision ID: 220856bbf43b
-Revises: 1dc0fee72dea
-Create Date: 2025-05-13 14:42:17.353614
-
-"""
-
-from typing import Sequence, Union
-
-import sqlalchemy as sa
-
-from alembic import op
-from letta.settings import settings
-
-# revision identifiers, used by Alembic.
-revision: str = "220856bbf43b"
-down_revision: Union[str, None] = "1dc0fee72dea"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def upgrade() -> None:
-    # Skip this migration for SQLite
-    if not settings.letta_pg_uri_no_default:
-        return
-
-    # add default value of `False`
-    op.add_column("block", sa.Column("read_only", sa.Boolean(), nullable=True))
-    op.execute(
-        """
-        UPDATE block
-        SET read_only = False
-    """
-    )
-    op.alter_column("block", "read_only", nullable=False)
-
-
-def downgrade() -> None:
-    # Skip this migration for SQLite
-    if not settings.letta_pg_uri_no_default:
-        return
-
-    op.drop_column("block", "read_only")