docs: publish an assertive repository overview

[Litju]

What changed

• Replaced the minimal public README with a concise, assertive description of SecScanMonitor.
• Added the mission, operating model, evidence-to-finding path, repository map, current capabilities, local setup, security boundaries, and qualification limits.
• Added the existing public product mark and an accessible Mermaid evidence-flow diagram.
• Closed with the roadmap statement that many features are still to come.

Why

The public repository needed a clear product explanation that communicates what is implemented today without implying hosted production qualification or unsupported capabilities.

Validation

• Public export built twice from source revision 7a37e2c9daafc2234c19364ad4424d78cde8786f.
• Both exports: 249 files, tree digest 9fe0da67d592ed7f38ea3cddc6bd89749d920971459b7b063c0189711d6b58ad.
• Exporter tests: 5 passed.
• SBOM test: 1 passed.
• Exported Python checks: 155 passed, 43 expected skips; the pre-existing side-effect-ledger integration case was isolated because it hangs in this Windows environment.
• No binary screenshots were added; excluded private screenshots contain non-public fixture names.