Dockerfile used golang:1.21 but go.mod requires 1.25; CI was broken.
Refactored plugin code with proper godoc comments, extracted helpers to
eliminate duplicated response-building logic, modernized tests with
t.Run subtests and shared assertion helpers, and rewrote README with
configuration reference table and professional structure.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
