Fix build and polish codebase to OSS quality

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>

Dockerfile

@@ -1,8 +1,9 @@
-# run go test inside a docker container for consistency as acceptance testing
-FROM golang:1.21
+# Dockerfile for CI: runs vet and tests inside a consistent Go environment.
+FROM golang:1.25
 
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN go test -v
+RUN go vet ./...
+RUN go test -v ./...