chore: fix readme and add copilot instructions

[GordonSmith]

Checklist:

• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit message includes a "fixes" reference if appropriate.
  • The commit is signed.
• The change has been fully tested:
  • I have viewed all related gallery items
  • I have viewed all related dermatology items
• There are no similar instances of the same problem that should be addressed
  • I have addressed them here
  • I have raised new issues to address them separately

Testing:

[Copilot]

Pull Request Overview

This PR updates the dataflow package documentation by correcting test file paths in the README and adding comprehensive Copilot instructions for package contributors.

• Updates all test reference links in README from old src/__tests__/*.ts paths to new tests/*.spec.ts paths
• Adds a new .github/copilot-instructions.md file documenting architecture, patterns, and development workflows

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/dataflow/README.md	Updates 28 test file reference links to point to the correct tests/*.spec.ts location after test directory restructuring
packages/dataflow/.github/copilot-instructions.md	Adds comprehensive documentation covering architecture, coding patterns, build workflows, and TypeScript configuration

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.