ci: split release workflow from PR checks

[striderZA]

Summary

Split the old combined GitHub Actions workflow into a PR-only CI workflow and a tag-only release workflow.

Related issue

N/A

Type of change

• Bug fix
• New feature
• Refactor
• Documentation
• Build / CI

Test plan

• cmake --build build
• ctest --test-dir build --output-on-failure -E "Benchmark|test_ui|ADC DDC output grid spans"
• Manual verification steps (if applicable):
  • Verified .github/workflows/ci.yml is pull_request-only and keeps format, lint-commits, build, sanitizer, and coverage
  • Verified .github/workflows/release.yml is push.tags: v*-only, keeps only Linux GCC 14 (Debug) + Windows MinGW-w64 artifact builds, preserves the tag/CMake version check, and skips ctest
  • Verified .github/workflows/build.yml was removed
  • Verified CONTRIBUTING.md now describes the split workflow contract

Checklist

• My code follows the existing code style (see .clang-format)
• I ran clang-format -i on changed files (N/A: no C++ file changes)
• I added or updated tests where appropriate (N/A: workflow/docs-only change; verified behavior via targeted checks)
• Existing tests still pass