Add copilot instructions for crypto backend implementation

[gdams]

Adds a .github/copilot-instructions.md that captures the architecture patterns, integration conventions, and review guidance for implementing crypto algorithms in the Microsoft Go backend system.

The instructions were derived from:

• Patch 0002 architecture (backend shim files, nobackend stubs, integration patterns for ECDSA, ML-KEM, Ed25519, ChaCha20Poly1305, etc.)
• @qmuntal's PR reviews scraped via gh CLI, including:
  • PR Wire ML-DSA into the crypto backend #2342 (ML-DSA): value vs pointer for backend keys, pushing operations into backends, caching reconstructed Go keys, adjusting allocation tests
  • PR Enable GODEBUG=fips140=only with systemcrypto backends #2149 (FIPS 140-only): keeping original variable names, FIPSApprovedHash as top-level function
  • PR Consolidate crypto backend GOEXPERIMENTs into systemcrypto #2126 (GOEXPERIMENT consolidation): docs requirements, error message guidance
  • PR openssl: delete cgo-less experiment and promote to default #2106 (cgo-less promotion): documentation needs
  • PR implement CSHAKE #1984 (CSHAKE): CrossPlatformCryptography.md update requirement

Covers:

• End-to-end git go-patch workflow (apply → edit → commit → rebase/squash → extract → verify)
• Backend shim patterns for all four files (linux/windows/darwin/nobackend)
• Algorithm integration patterns (gate functions, dispatch, fallback)
• Build tag and import conventions
• FIPS 140-only mode enforcement
• Documentation generation via updatecryptodocs
• Patch separation rules (vendor in 0001, crypto in 0002)

[Copilot]

Pull request overview

Adds a new .github/copilot-instructions.md intended to guide contributors (and Copilot) when implementing or modifying crypto algorithms in the Microsoft Go “systemcrypto” backend architecture, including the git go-patch workflow, backend shim patterns, gating/dispatch guidance, FIPS 140-only considerations, and doc generation steps.

Changes:

• Introduces a crypto-backend-focused Copilot instructions document under .github/.
• Documents the expected git go-patch apply/edit/rebase/extract/commit flow for patch maintenance.
• Captures backend integration patterns (shims, gates, dispatch, fallback) and related conventions (build tags, import aliasing, doc generation).

Show a summary per file

File	Description
.github/copilot-instructions.md	Adds repository-specific guidance for crypto backend development workflow and integration conventions.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2

[gdams]

@qmuntal updated PTAL