Prepare repository for public OSS release

[yogayulanda]

Summary

Prepares go-core for a professional public OSS release by improving repository hygiene, onboarding documentation, security guidance, and public-facing project metadata.

Changes

• Reworked README.md into a concise public OSS onboarding guide.
• Added Apache-2.0 LICENSE.
• Added CONTRIBUTING.md.
• Added root SECURITY.md.
• Added safe .env.example.
• Updated .gitignore for env files, local build/test outputs, editor noise, local certificates, and assistant-local artifacts.
• Removed legacy .ai/ prompt/context artifacts and duplicate context files.
• Preserved .forge/ and CLAUDE.md for Forge context workflow.
• Sanitized release-gate examples to avoid private topology assumptions.
• Replaced product-flavored test fixture naming with neutral example naming.

Validation

• go test ./... passed.
• go vet ./... passed.
• Current-tree hygiene scan did not find real secrets.
• Fallback git-history scan did not find high-confidence real secrets; only historical placeholder/example values were detected.

Notes

This PR intentionally does not rewrite git history. The scan did not indicate real credential leakage, and avoiding history rewrite preserves commit stability.