Add PDF/SVG exports, wire up tooltip a11y, drop leftover footer branding

[Hefrock]

Summary

Closes out the remaining items from the last gap analysis:

• scripts/export-pdf.js — prints index.html to exports/ai-agent-patterns.pdf via Playwright. Added a @media print block to index.html that hides the toolbar and keeps cards from splitting across page breaks.
• scripts/export-svg.js — extracts each pattern's flow diagram as a standalone exports/svg/pattern-{slug}.svg. Bakes every element's resolved fill/stroke/font-*/filter via getComputedStyle and inlines the shared #arrow marker + #nodeShadow filter <defs>, so each file opens correctly with zero dependency on the page's stylesheet or CSS custom properties.
• Both wired into package.json (npm run export:pdf / export:svg) and into ci.yml's PR validation smoke test.
• Statefulness tooltip a11y: the meta value now has aria-describedby pointing at a visually-hidden (.sr-only) span carrying the same explanation text, since the CSS ::after tooltip content isn't reliably exposed to screen readers.
• Removed leftover "Built with Claude" footer text — found while visually reviewing the new PDF export; this looks like what "remove your branding at the bottom" was originally asking for and it had slipped through.
• Updated CLAUDE.md and README.md to document all of the above.

Test plan

• npm run validate passes (15/15 patterns)
• Regenerated and visually inspected all 3 rendered PDF pages (toolbar hidden, no clipped cards, all colors/pills correct)
• Regenerated all 15 SVGs; rendered 3 standalone (loop, parallel-cluster, branch-cluster) directly from disk in a browser with no other files present — all render pixel-correct with proper arrowheads, drop shadows, and dashed styling
• Verified aria-describedby resolves to the correct hidden text for a sample card
• Re-ran export:images / export:pptx after the footer/CSS edits — no regressions

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