[Docs] Fix copilot-instructions and script comments data path drift

Problem summary

.github/copilot-instructions.md tells contributors datasets live in assets/data/, but the app and architecture docs use data/vocabularies/. This causes incorrect guidance for AI agents and new contributors.

Affected area

• .github/copilot-instructions.md
• scripts/fetch-data.js (stale header comment)
• DATA_SOURCES.md, docs/architecture.md

Expected behavior

All contributor and agent docs reference the same canonical vocabulary path.

Actual behavior

Conflicting paths across docs increase the risk of fixes landing in the wrong directory.

Acceptance criteria

• copilot-instructions.md references the canonical data path.
• Stale comments in scripts/fetch-data.js are corrected.
• Cross-doc audit confirms no remaining assets/data/ references for runtime vocabularies (or documents intentional legacy use).

Start by reviewing .github/copilot-instructions.md, scripts/fetch-data.js, DATA_SOURCES.md, and docs/architecture.md, then search the repository for assets/data/ references. Update the contributor and script guidance to use the canonical data/vocabularies/ path, and confirm the audit finds no conflicting runtime-vocabulary references or that any intentional legacy use is documented.