Fix readme and docs to reflect provider/model syntax

[anakin87]

Description

When using the latest stable version of Verifiers (v0.1.9.post3), running prime eval run my-env -m gpt-5-nano fails with:

Invalid model: Model 'gpt-5-nano' not found or unavailable (GET https://api.pinference.ai/api/v1/models/gpt-5-nano → 404).

Using prime eval run my-env -m openai/gpt-5-nano works instead.

So I thought of updating README and docs. I also made some other small fixes.

@willccbb

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Low Risk
Docs-only changes (plus a README template string) with no runtime behavior impact; risk is limited to potential confusion if examples are still incomplete or mismatch the CLI.

Overview
Documentation examples for prime eval run are updated to use the provider-qualified model format (e.g. -m openai/gpt-4.1-mini / openai/gpt-5-nano) instead of bare model names, aligning with current CLI behavior.

Fixes minor doc inconsistencies by qualifying Rubric as vf.Rubric in example code and updating README links to point at the docs/ paths; the environment init README template is updated to match the new model syntax.

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[anakin87]

@willccbb @mikasenghaas this should still be relevant. Right?

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