refactor(auxiliary): move WTK-AUXILIARY.md template into file

[nettee]

Summary

• move the static WTK auxiliary guidance prose into a dedicated template file
• keep Rust responsible only for rendering the dynamic auxiliary ref and target entries
• add a focused unit test to verify the template placeholder is populated

Validation

• cargo test render_guidance_uses_template_file
• cd e2e && uv run pytest test_auxiliary_group.py

Closes #73

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