fix: scope CodeQL to maintained source

[ekkolon]

Both open alerts originate in generated third-party code: undici uses protocol-mandated SHA-1 for WebSocket handshakes (RFC 6455), and is-unsafe matches the XML --> comment terminator. Neither rule identifies a flaw in setup-zig's own signature verification.

Scan src/, scripts/, and test/ with the pinned CodeQL workflow; leave dist/ to reproducible-build checks, dependency audits, and the SBOM. Add a regression check for the scan scope.

Before enabling SARIF uploads, switch off GitHub's CodeQL default setup in Settings → Advanced Security; it currently rejects results from this advanced workflow. No suppression of first-party findings.

[github-advanced-security]

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