docs(skills): add retrieval models skill

[oliverholworthy]

What does this PR do ?

Adds a retrieval-models agent skill for NeMo AutoModel bi-encoder and cross-encoder work, including guidance for retrieval wrappers, bidirectional backbones, recipes, collator shape contracts, validation, and live skill evals.

Changelog

• Add skills/retrieval-models/SKILL.md with retrieval model workflows, trigger boundaries, validation commands, and troubleshooting guidance.
• Add skills/retrieval-models/PITFALLS.md for common retrieval wrapper, metadata, and recipe shape issues.
• Register the retrieval skill files in packaging metadata and the skills README.
• Add skills/retrieval-models/evals/evals.json for astra live-agent evaluation coverage.

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you add or update any necessary documentation?

Additional Information

Validation run:

• astra-skill-eval evaluate skills/retrieval-models --static --markdown -> A, 100/100, no issues
• astra-skill-eval validate skills/retrieval-models -> all checks passed
• astra-skill-eval evaluate skills/retrieval-models --agent-eval -a codex --env-mode local --n-attempts 1 --skip-baseline --copy-repo --results-dir /tmp/astra-retrieval-models-eval-clean --markdown -> pass@1 3/4, accuracy 1.00
• Focused retrieval unit subset passed with one environment-sensitive flash-attn case deselected: 29 passed, 1 deselected

Draft PR because the live eval still shows one implementation-heavy scenario below threshold and should get a reviewer pass before marking ready.

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