docs(readme): rewrite for accuracy against v0.3 and a professional, visual layout

Bring the README in line with the shipped v0.3 engine and give it a
top-tier, GitHub-native presentation.

Accuracy fixes (stale v0.2 claims removed):
- CVE sources are OSV + NVD only (drop AVID / GitHub Advisory)
- reports are timestamped under dsgai-reports/ (not DSGAI-report.html at root)
- redaction is the V1-V3 rg -o --replace '' protocol with file IDs (not V1-V6)
- GitHub Action is the two-job, secret-less, SARIF-native workflow (no ANTHROPIC_API_KEY)
- document SARIF/Code Scanning, 5 CLI subcommands, 6 CVE ecosystems,
  gitleaks + Semgrep packs, suppressions/baseline/--diff, C#/Rust/Ruby

Presentation:
- centered masthead (title, tagline, badges, nav), TOC, feature grid
- redrawn mermaid showing the deterministic core vs optional LLM layer
- collapsible reference (controls, checkpoint, PDF, scope, tiers)
- GitHub alerts, a 'Why this scanner' positioning section
- OWASP + Emmanuel Guilherme Junior / Harish Ramachandran attribution preserved

Verified: internal links resolve, no-overclaim gate clean, all referenced
paths exist, suppression/CLI facts checked against the code.

Author: emmanuelgjr