docs: add openssf silver badge compliance documents

[rhuanbarreto]

Summary

• Add ROADMAP.md — public-facing roadmap through Phase 3 (ecosystem growth), derived from internal strategic/tactical plans. Satisfies documentation_roadmap (MUST).
• Add ASSURANCE-CASE.md — security assurance case with threat model (7 threats), trust boundary diagram, secure design principles, and countermeasures. Satisfies assurance_case (MUST).
• Add MAINTAINERS.md — maintainer list, access continuity plan with all critical access points and backup mechanisms. Satisfies access_continuity (MUST) and documents bus_factor (SHOULD).
• Add DCO enforcement — .github/workflows/dco.yml workflow + DCO section in CONTRIBUTING.md with sign-off instructions. Satisfies dco (SHOULD).
• Add OpenSSF Best Practices badge to README.
• Fix stale credential references — updated 11 doc files (EN + PT-BR) across security guide, plugin guides, and CLI reference to reflect current implementation: tokens stored exclusively in OS credential manager via git credential approve, no plain-text files on disk.

After merging, update the badge application with URLs to the new documents and mark internationalization as N/A (developer CLI tool; docs already i18n'd with EN + PT-BR).

Test plan

• Verify ROADMAP.md renders correctly on GitHub
• Verify ASSURANCE-CASE.md trust boundary diagram renders in monospace
• Verify MAINTAINERS.md renders correctly
• Verify DCO workflow syntax is valid (check Actions tab after push)
• Verify CONTRIBUTING.md DCO section has correct sign-off instructions
• Verify credential references are updated in all EN and PT-BR docs
• Verify OpenSSF badge renders in README

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