docs: document coding standards enforcement

[hittyt]

Summary

• Document OpenSandbox coding standards and the automated enforcement points used by Ruff, ESLint, Spotless/ktlint, .NET analyzers, go vet, golangci-lint, and read-only gofmt checks.
• Add the missing C# code-interpreter SDK .editorconfig so the documented C# style is consistently enforced.
• Document build-system standards for native Go binaries and update Makefile/Docker build paths to preserve caller-provided build variables while appending project flags.
• Make native Go binary builds more repeatable with -trimpath, -buildvcs=false, -buildid=, -B none, and SOURCE_DATE_EPOCH/fixed metadata support.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification
  • test -z "$(gofmt -l components/execd components/ingress components/egress components/internal kubernetes sdks/sandbox/go)"
  • workflow YAML parse check with Ruby YAML.load_file
  • bash -n components/execd/build.sh components/egress/build.sh components/ingress/build.sh kubernetes/build.sh
  • docker build --check for execd, ingress, egress, and kubernetes Dockerfiles
  • Makefile dry-runs verified caller GOFLAGS/LDFLAGS are preserved and project flags are appended
  • repeatability checks built each native Go binary twice with fixed metadata and compared bytes
  • git diff --check

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered