Merge repositories in new branch

[uniqueg]

Merge tesk-core and TESK-api repositories into a new branch main in this directory, keeping all directory structures. Make sure all currently available files and all commit histories are preserved. In case of conflicts, prioritize TESK and tesk-coreovertesk-api`; otherwise choose your best judgement to resolve conflicts.

From that point forward, work only on the main branch (we will make it the default branch once everything is working).

See here for a possible approach that should satisfy all requirements.