chore: update Go from 1.26.0 to 1.26.4

[Shelnutt2]

Summary

Updates the Go version in go.mod from 1.26.0 to 1.26.4 to pick up security patches from the Go 1.26.4 release.

CVEs addressed

• CVE-2026-42504: Quadratic complexity in mime.WordDecoder.DecodeHeader (DoS via crafted MIME headers)
• CVE-2026-42507: net/textproto log injection via unescaped error inputs
• CVE-2026-27145: Quadratic complexity in crypto/x509.Certificate.VerifyHostname

Closes #515

Generated by Coder Agents (ENG-2854)

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