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>

CITATION.cff

@@ -1,25 +0,0 @@
-cff-version: 1.2.0
-message: "If you use this software, please cite it as below."
-title: "Letta"
-url: "https://github.com/letta-ai/letta"
-preferred-citation:
-  type: article
-  authors:
-  - family-names: "Packer"
-    given-names: "Charles"
-  - family-names: "Wooders"
-    given-names: "Sarah"
-  - family-names: "Lin"
-    given-names: "Kevin"
-  - family-names: "Fang"
-    given-names: "Vivian"
-  - family-names: "Patil"
-    given-names: "Shishir G"
-  - family-names: "Stoica"
-    given-names: "Ion"
-  - family-names: "Gonzalez"
-    given-names: "Joseph E"
-  journal: "arXiv preprint arXiv:2310.08560"
-  month: 10
-  title: "MemGPT: Towards LLMs as Operating Systems"
-  year: 2023