docs: refresh README with mermaid diagram and improved layout

[MattDevy]

Summary

• Replaces the ASCII flow diagram with a Mermaid flowchart LR showing the session/analyzer data flow
• Adds CI, npm, and license badges
• Converts prose-heavy sections to tables (requirements, confidence scoring, safety features, quality tiers, graduation targets, config reference)
• Tightens the overall structure with --- section dividers and cleaner headings
• No content removed — same information, better visual hierarchy

BEGIN_COMMIT_OVERRIDE
fix: refresh docs
END_COMMIT_OVERRIDE

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
⚠️ MARKDOWN	markdownlint	1		2	0	0.63s
✅ REPOSITORY	gitleaks	yes		no	no	0.85s
✅ REPOSITORY	git_diff	yes		no	no	0.34s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 2 errors

README.md:267 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
README.md:327 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]

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