Add GOVERNANCE.md

[sirdeggen]

Problem

go-stack has CODEOWNERS and SECURITY.md but no GOVERNANCE.md.

The equivalent ts-stack repo has a GOVERNANCE.md defining:

• Roles (Stack Architect, Domain Leads, Language Lead, Security Lead, QA/Release)
• Decision-making rules (day-to-day vs major refactor vs breaking change)
• Versioning and release cadence
• PR requirements checklist and review SLA
• Parity classes (Required / Intended / Best-effort / Unsupported)

Task

Add GOVERNANCE.md to go-stack root, adapted from ts-stack/GOVERNANCE.md:

• Replace TS-specific tooling references (changesets → Go module tags)
• Add Go-specific release cadence (per-module semver tags)
• Keep parity class definitions consistent with ts-stack

Acceptance

• GOVERNANCE.md exists at repo root
• Content is accurate for a Go workspace monorepo
• Consistent with ts-stack governance where cross-language rules apply