Add PR validation workflow (schema check + export smoke test)

[Hefrock]

Summary

• Adds scripts/validate-patterns.js: loads index.html and checks every PATTERNS entry against the schema documented in CLAUDE.md — required fields, valid cat/group/kind enums, members present when kind is set, no duplicate pattern names, and that every distinct statefulness value has a matching STATEFULNESS_INFO entry (so the hover tooltip from Add statefulness explanation tooltips #2 never silently renders empty)
• Adds .github/workflows/ci.yml, triggered on every PR targeting main. Runs npm run validate, then npm run export:images and npm run export:pptx as an end-to-end smoke test of the real export scripts
• Documents both in CLAUDE.md

Why

Until now there was no PR-time validation in this repo — deploy.yml only runs after merging to main. A real bug (the loop-pattern "Done" node clipping, fixed earlier) shipped past manual review and was only caught by deliberately running the export scripts by hand. This gives every future PR an automated check before merge.

Limitations

This catches crashes and schema violations, not visual regressions (e.g. a node clipped at the edge of its SVG viewBox) — those still need a manual screenshot check.

Test plan

• Ran validate-patterns.js against the current index.html — passes (15/15 patterns)
• Deliberately broke the data three different ways (invalid cat enum, kind:"parallel" step missing members, a statefulness value with no STATEFULNESS_INFO entry) and confirmed each is caught with a clear error message and non-zero exit code
• Confirmed npm ci works (lockfile present) and npm run export:images / npm run export:pptx still run clean against current index.html

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