refactor: move core server code into core/ directory

[Blackgaurd]

Summary

• Moves 6 source directories (api, calendars, db, runtime, service, utils) into builders/server/core/
• Updates all internal imports across source, test, benchmark, and integration files to use core. prefix
• Fixes @patch decorator strings and patch() context manager calls to reference new module paths
• Updates WORKER_PATH in core/runtime/runner.py for the new directory depth
• All 302 tests pass

[github-actions]

• main
  • refactor: create core/ package init #343
    • refactor: move core server code into core/ directory 👈
      • refactor: move core tests into tests/core/ directory #345