fix: keep catalog skill references self-contained

[danielAsaboro]

Summary

• link the canonical DataHub CLI reference into each catalog skill's local references/ directory
• update the five affected skills to use only skill-local reference paths
• add an installation-integrity regression and run it from pre-commit
• clarify Skills CLI versus manual source-copy behavior in the README

Problem

The five catalog skills cited ../shared-references/datahub-cli-reference.md. The documented npx skills add datahub-project/datahub-skills flow discovers and installs the skill directories, but does not install that sibling directory. This leaves an installed skill with a reference that escapes its installation root and points to a missing file.

The source tree now keeps one canonical reference document while skill-local symlinks let the Skills CLI dereference that document into each standalone installation.

Validation

• python3 -m unittest tests/test-catalog-skill-installation.py -v
• all non-Node upstream pre-commit hooks, including symlink checks, Ruff, and the new installation-integrity hook
• npx --yes markdownlint-cli2@0.21.0 --config .markdownlint-cli2.yaml '**/*.md' '#node_modules' (165 files, 0 errors)
• npx --yes prettier@4.0.0-alpha.8 --check ...
• git diff --check

The model-driven tests/run-tests.sh suite could not start locally because the Claude CLI test dependency is not authenticated; it exited before executing its first assertion.