Fix: Avoid hard-coded Cohere API key in RAG skill

[snvtac]

Summary

• Replace the hard-coded Cohere API key placeholder in the RAG reranking example with COHERE_API_KEY from the environment.
• Add a short note to load provider keys from environment variables or a secrets manager and avoid committing API keys.

Impact

This keeps the documentation example aligned with safer credential handling while preserving the existing reranking flow.

Fixes #210.

Validation

• git diff --check
• UV_CACHE_DIR=../uv-cache UV_PYTHON_INSTALL_DIR=../uv-python uv run --python 3.11 --with pyyaml python scripts/validate-skills.py --skill rag-architect
• UV_CACHE_DIR=../uv-cache UV_PYTHON_INSTALL_DIR=../uv-python uv run --python 3.11 --with pyyaml python scripts/validate-skills.py
• UV_CACHE_DIR=../uv-cache UV_PYTHON_INSTALL_DIR=../uv-python uv run --python 3.11 --with pyyaml python scripts/validate-markdown.py --check
• UV_CACHE_DIR=../uv-cache UV_PYTHON_INSTALL_DIR=../uv-python uv run --python 3.11 --with pyyaml python scripts/update-docs.py --check
• ./site/node_modules/.bin/prettier --check skills/rag-architect/SKILL.md
• ./site/node_modules/.bin/prettier --check "skills/**/*.md" "docs/**/*.md" "*.md" "research/**/*.md"