[tx] Update README.md and move tx over to skyrl/ folder

[pcmoritz]

This completes #1145 for the skyrl-tx folder.

For users, all that is required to update is to run in the SkyRL base folder, and replace -m tx.tinker.api with -m skyrl.tinker.api in the command line. See the updates to the README: https://github.com/NovaSky-AI/SkyRL/pull/1226/changes#diff-3ab5f33c37f9cfd1a0e08eef29bf8a2fcbde1b7e140b49f966b3740e82b43f3bL61

The code that was previously in the skyrl-tx folder can now be found in skyrl/{backends, tinker, tx, utils}.

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[pcmoritz]

/gemini review

[gemini-code-assist]

Code Review

This pull request is a significant refactoring that moves the tx package into the skyrl/ folder, unifying the codebase. The changes primarily involve removing files from the old skyrl-tx directory and updating the README.md to reflect the new package structure. The command updates in the README are correct. However, I've identified an incorrect GitHub link in the README that will be broken after the refactoring and have suggested a fix. Overall, this is a good step towards a more organized repository structure.

[gemini-code-assist]

The GitHub link seems to be incorrect. Since the skyrl-tx folder is being moved to skyrl/, this link will be broken. It should probably point to the new skyrl directory, and the link text should be updated accordingly.

Suggested change

	- **GitHub**: [NovaSky-AI/SkyRL/skyrl-tx](https://github.com/NovaSky-AI/SkyRL/tree/main/skyrl-tx/README.md)
	- **GitHub**: [NovaSky-AI/SkyRL/skyrl](https://github.com/NovaSky-AI/SkyRL/tree/main/skyrl)

[pcmoritz]

The README.md file is actually kept around :)