Add Copilot instructions for AGENTS.md workflow

[napetrov]

Add .github/copilot-instructions.md to establish the AGENTS initialization/precedence workflow, aligned with the pattern used in mkl_random and mkl_fft:

• defines precedence: copilot-instructions > nearest AGENTS > root AGENTS
• mandates root+nearest AGENTS lookup flow
• adds source-of-truth guardrails for build/CI/API/test files
• encodes mkl_umath-specific constraints (icx/oneMKL, patch API stability, numerical correctness)

This is docs-only (no code changes).

[ndgrigorian]

• encodes mkl_umath-specific constraints (icx/oneMKL, patch API stability, numerical correctness)

with meson-python PR open, in there, I've added changes that let us build with standard clang and added workflow to test with standard clang. All that was needed was to gate Intel-only flags.

so, I think we can remove icx constraint.

[antonwolfy]

LGTM