CI hardening: CodeQL + concurrency guard

[varkart]

Changes

• ci.yml — add concurrency group per ref with cancel-in-progress so superseded pushes stop early
• codeql.yml (new) — CodeQL v4 static analysis on push/PR to main plus weekly Monday cron

Notes

• cache: npm on setup-node was considered but skipped — repo has no package-lock.json, which setup-node caching requires. Committing a lockfile (and moving to npm ci) is a good follow-up.
• Dependabot config already present, untouched.

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