Add portable dbt-tools CLI investigation skills (status, discover-search, deps, explain-impact)

[yu-iskw]

Motivation

• Provide portable, documentation-style Agent Skills so AI coding agents can reliably use the installed dbt-tools CLI for artifact investigation without adding runtime wrappers.
• Surface stable, JSON-first, agent-friendly command patterns and failure-handling advice rather than duplicating the full CLI reference so skills remain robust across CLI surface changes.

Description

• Added four independent skills under plugins/dbt-tools-cli/skills/: status, discover-search, deps, and explain-impact, each containing a SKILL.md with YAML frontmatter (name, description, compatibility) and a self-contained workflow (trigger scenarios, inputs, recommended command pattern, interpretation, failure handling, verification).
• For each skill added a lightweight agent-oriented cheat sheet at references/commands.md with JSON-first examples, output-bounding guidance, and common failure responses.
• Updated plugins/dbt-tools-cli/README.md to list the new skills so agents can discover them from the plugin index.
• All changes are documentation-only and confined to plugins/dbt-tools-cli/ (no CLI implementation, code, or new dependencies were added).

Testing

• Ran ./plugins/tests/verify-agent-plugins.sh structural and it completed successfully.
• Ran unit tests via pnpm test and they passed (317 files, 2751 tests reported).
• Ran pnpm lint:report and it produced a clean lint report (score 100).
• Built the workspace with pnpm build to satisfy a knip config-loading prerequisite and then re-ran pnpm knip, which succeeded with configuration hints.
• Ran coverage via pnpm coverage:report and it completed and wrote coverage-report.json.
• pnpm exec trunk check ... on the touched Markdown paths failed in this environment due to a network ENETUNREACH error when Trunk tried to fetch remote resources; this is an environment/network failure and not a repo-content error.

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Codex Task