Add Copilot instructions for repository

[Copilot]

Configures GitHub Copilot coding agent instructions as per best practices documentation.

Changes

• Added .github/copilot-instructions.md with repository-specific context:
  • Project architecture: Python 3.11+ Sentry auth plugin using OAuth2/OIDC
  • Development workflow: Poetry setup, git submodule for Sentry dev dependencies
  • Code standards: Black, isort (Black profile), flake8
  • Testing: pytest with @control_silo_test decorator pattern
  • Key patterns:
    • OIDC auto-discovery vs manual endpoint configuration
    • Sentry version compatibility shims (apps.py, views.py)
    • Identity migration via MigratingIdentityId (email → sub)
    • DATA_VERSION for config schema changes
  • Constraints: backward compatibility, no PII logging, minimal changes

Provides Copilot with domain knowledge to improve code suggestions and reduce context-gathering overhead.

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.

</issue_description>

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

• Fixes ✨ Set up Copilot instructions #72

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

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