Add SECURITY.md, fix workflow permissions, redesign README

[sebastientaggart]

Polish the project's GitHub presence with four changes:

• SECURITY.md: Add a lightweight security policy with GitHub Issues reporting and a private-disclosure carve-out for supply-chain concerns
• Workflow permissions: Add explicit permissions: { contents: read } to both sync-check.yml and test.yml, resolving the CodeQL "workflow does not contain permissions" warnings
• README redesign: Restructure the header with centered title, tagline, badge row, and hero image (LiteLLM-style layout) while preserving all existing content

Issue #130

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

No issues found. Code looks correct and follows project conventions.

• The permissions: { contents: read } additions to both workflow files correctly apply least-privilege and resolve the CodeQL warnings.
• SECURITY.md is reasonable for the project scope and correctly distinguishes public vs. private disclosure paths.
• README restructure preserves all existing badge links and content; only layout changes.