Add Copilot instructions for repository

[Copilot]

Establishes AI coding agent guidelines following vendor-neutral standards for the repository.

Changes

• Added .github/copilot-instructions.md (205 lines) with:
  • Project structure and module responsibilities (fhiry.py, fhirsearch.py, bqsearch.py, etc.)
  • Development workflow: uv package manager, make targets (make test, make check, make build)
  • Code standards: Ruff formatting (120 char), mypy type hints (strict), Google-style docstrings
  • Git Flow strategy: target develop branch, squash commits before PR
  • FHIR domain context: resource types, flattening logic, coding systems (SNOMED, LOINC)
  • Complete dependency list with purposes (pandas, google-cloud-bigquery, tqdm, click, etc.)
  • Testing expectations: pytest with coverage, fixtures in tests/conftest.py
  • Pre-commit hooks: ruff, yaml/json/toml validation

Format

Standard Markdown structure usable by any AI coding assistant, not GitHub Copilot-specific.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository. Use a vendor-neutral standard for providing instructions to AI coding agents whenever possible.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #236

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.3%. Comparing base (8835b94) to head (38e3f23).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #237   +/-   ##
=======================================
  Coverage     59.3%   59.3%           
=======================================
  Files            9       9           
  Lines          633     633           
  Branches       122     122           
=======================================
  Hits           376     376           
  Misses         203     203           
  Partials        54      54           

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