docs: update custom-agents.md and prompts.md with cross-artifact reference portability rules

[github-actions]

Component: Documentation

Bug Description:

The commit db1be8f ("refactor(skills): make cross-artifact references portable") added a ## Referencing Other Artifacts section to .github/instructions/hve-core/hve-builder.instructions.md and a ## Path Portability section to docs/contributing/skills.md. However, the equivalent contributing guides for agents and prompts were not updated with this guidance.

Neither docs/contributing/custom-agents.md nor docs/contributing/prompts.md documents the new cross-artifact reference rules:

• Refer to a skill, agent, subagent, or prompt by the name: value from its frontmatter, not by a hard-coded path.
• Refer to an instruction file by its full <name>.instructions.md filename.
• Reserve file paths for a skill's own bundled resources (relative to skill root only).
• Never hard-code a skill's SKILL.md path or a cross-skill directory path.
• Use #file: only when the prompt must pull in another file's full contents.

Expected Behavior:

docs/contributing/custom-agents.md and docs/contributing/prompts.md should each have a section (equivalent to docs/contributing/skills.md § Path Portability) documenting these cross-artifact reference portability rules so authors of agents and prompts receive the same guidance as skill authors.

Steps to Reproduce:

Introduced by PR #2599 / commit db1be8f.

Additional Context:

• New rule source: .github/instructions/hve-core/hve-builder.instructions.md § "Referencing Other Artifacts" (lines ~194–211)
• Skills doc updated: docs/contributing/skills.md § "Path Portability" (line ~513)
• Agents doc missing guidance: docs/contributing/custom-agents.md — no portability or cross-artifact reference section
• Prompts doc missing guidance: docs/contributing/prompts.md — no portability or cross-artifact reference section
• Related open issue: fix: Distributed AI artifacts reference hardcoded workspace-root paths that break in extension and peer-clone installs #741 (tracks the broader operational path portability problem across artifacts)

Generated by Documentation Update Check · 78.7 AIC · ⌖ 6.09 AIC · ⊞ 7.3K · ◷

[nightcityblade]

Hi, I'd like to work on this. I'll submit a focused documentation PR shortly.