Make the installed TRL skill discoverable

[DaoyuanLi2816]

What does this PR do?

Moves TRL's packaged skill to the installed-library convention at trl/.agents/skills/trl/SKILL.md, where generic dependency scanners can discover it without copying a stale snapshot.

The skill is rewritten around the Python API, dataset contracts, custom reward functions, and the settings that materially affect correctness or throughput. This also removes the per-library trl skills installer and its implementation/tests; standard training CLI commands remain unchanged.

The wheel and sdist both include the new skill path.

Fixes #6743

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue? See Make TRL's skill discoverable from an install, and drop trl skills install #6743.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Validation:

• 9 focused CLI and packaged-skill tests passed
• Ruff check and format check passed for changed Python files
• sdist and wheel built successfully
• wheel contains trl/.agents/skills/trl/SKILL.md and no legacy trl/skills/ files
• all Python API fields referenced by the skill were checked against current main

AI writing disclosure

• No AI usage: the PR was written entirely by a human.
• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.
• AI-generated: the PR was mostly or fully generated by an AI tool.

Who can review?

Anyone in the community is free to review the PR once the tests have passed.

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Note

Medium Risk
Removes a public CLI subcommand and Python APIs (trl.skills), which is a breaking change for anyone using trl skills; training CLIs and core library behavior are unchanged.

Overview
Packaged skill location and content — The wheel/sdist now ship trl/.agents/skills/**/* (via MANIFEST.in) instead of trl/skills/**/*.md. A single skill lives at trl/.agents/skills/trl/SKILL.md (name trl), focused on the Python API (trainers, dataset columns, GRPO rewards, throughput/correctness knobs) rather than the old CLI-centric trl-training skill.

Removed custom skills tooling — The entire trl/skills package (skills.py, cli.py), SkillsCommand, and trl skills list/install/uninstall flow are deleted. get_commands() no longer registers skills; ruff’s per-file ignore for trl/skills/cli.py is dropped.

Tests — tests/test_skills.py and tests/test_skills_cli.py are removed. tests/test_packaged_skill.py now asserts the installed-library skill path/metadata and that no skills CLI command is registered.

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