chore: add Copilot instructions for AI coding agents

[Copilot]

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

Changes

• Added .github/copilot-instructions.md - Comprehensive guidance covering:
  • Repository structure (Auth, Functions, PostgREST, Realtime, Storage modules)
  • Build/test commands for SPM and Xcode across platforms
  • Swift 6 conventions (strict concurrency, Sendable conformance, async/await patterns)
  • Testing patterns (XCTest, snapshot testing, Mocker for URLSession)
  • Conventional Commits workflow with release-please
  • CI/CD matrix (macOS/iOS/tvOS/watchOS/visionOS, multiple Xcode versions)
  • Support policy (4 latest major versions per platform)

This provides AI agents with actionable context about the Swift SDK's architecture, development workflow, and quality standards.

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 #842

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