feat: add root README.md #19

Description

Add a root README.md describing the purpose of this repository and how it manages the unbound-force GitHub organization.

Acceptance Criteria

• README.md created at repo root
• Documents two complementary tools: Peribolos (org/config.yaml) and safe-settings (safe-settings/)
• Quick Start section with prerequisites and validation commands
• References section linking to Peribolos and safe-settings documentation
• Links to MAINTAINING.md for operational workflows
• Adapted for unbound-force (not a copy of zero-dot-force branding)

References

• Modeled on: zero-dot-force/.github/README.md