docs: run the actual skill suite in contributor test instructions

[xiehuanyi]

Summary

The contributor guide uses pnpm --filter @understand-anything/skill test, but that package script only prints a reminder and does not run tests. Replace it with the root pnpm test command. Clarify that both the core command and root command are required for complete coverage, because the root Vitest config excludes core tests, and align the PR checklist with that split.

Linked issue(s)

Documentation correction verified against understand-anything-plugin/package.json, the root vitest.config.ts and .github/workflows/ci.yml.

How I tested this

• pnpm lint
• pnpm --filter @understand-anything/core test — 47 files, 1,013 tests passed
• pnpm test — 38 files passed, 1 skipped; 786 tests passed, 12 skipped
• Inspected package scripts and CI commands, and passed git diff --check

Versioning

• N/A — documentation-only change

AI assistance: prepared with Codex and checked against the actual package scripts and both test commands.

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