Add governance files (CONTRIBUTING, CODE_OF_CONDUCT, SECURITY, MAINTAINERS)

[renecannao]

Phase 1, Task 3

Part of: Phase 1 — Identity & Trust (4.0.0-rc1)
Plan: docs/superpowers/plans/2026-03-23-phase1-identity-and-trust.md

Description

Create root-level governance files to establish community contribution norms.

Files to Create

• CONTRIBUTING.md — How to file issues, submit PRs, coding standards (gofmt, tests), DCO sign-off requirement
• CODE_OF_CONDUCT.md — Contributor Covenant v2.1
• SECURITY.md — Vulnerability reporting process (email, not public issues)
• MAINTAINERS.md — Current maintainers and roles

Acceptance Criteria

• All four files exist at repo root
• CONTRIBUTING.md covers DCO sign-off, gofmt, CI requirements
• SECURITY.md has a contact email for vulnerability reports
• CODE_OF_CONDUCT.md uses Contributor Covenant v2.1

[renecannao]

Done in PR #12