chore: Add Copilot instructions for repository

[Copilot]

Adds .github/copilot-instructions.md to configure GitHub Copilot coding agent with repository-specific context and conventions.

Contents:

• Project architecture: Cargo workspace structure, NodeJS code generation (doT.js templates), async/thread-safe design
• Development setup: MSRV 1.71.1, API key configuration, build/test commands with feature flags
• Code standards: #![forbid(unsafe_code)], rustfmt config, clippy enforcement, Wasm compatibility
• Feature flags: nightly, TLS backends (default-tls/native-tls/rustls-tls), tracing, metrics, deny-unknown
• Testing: Integration tests requiring RGAPI_KEY, wasm-pack testing, feature flag combinations
• CI/CD: GitHub Actions workflows for checks, lints, docs, MSRV verification
• Common workflows: Adding endpoints/models via srcgen, dependency updates, bug fixes

Enables Copilot to provide context-aware suggestions aligned with project conventions and constraints.

Original prompt

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This section details on the original issue you should resolve

<issue_title>chore: ✨ 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.

<Onboard this repo></issue_description>

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

• Fixes MingweiSamuel/Riven#102

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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.