docs(contributing): document artifact path portability

[Shaurya Saria (icecold009)]

Summary

• Document cross-artifact reference portability rules in the custom agents guide.
• Document the same rules in the prompts guide, including the constrained use of #file:.
• Align both guides with the existing HVE Builder portability guidance.

Closes #2662

Testing

• markdownlint-cli2 docs/contributing/custom-agents.md docs/contributing/prompts.md — passed.
• cspell docs/contributing/custom-agents.md docs/contributing/prompts.md — passed.
• npm run lint:tables — passed.
• git diff --check — passed.
• npm run lint:frontmatter — blocked locally because the required PowerShell-Yaml module is not installed; the validator reports ConvertFrom-Yaml cmdlet not found across the checkout.

[Jamie Kim (jkim323)]

Thank you Shaurya Saria (@icecold009) for working on this PR and contributing. The issue has been addressed by another PR so please go ahead and close this.