refactor: improve Lambda handler and update Go to 1.25.5

- Update Dockerfile: Go 1.25.5, fix build context paths
- Use git archive instead of zip to exclude .git directory
- Add context support for git/zip operations (timeout control)
- Optimize git clone (--depth 1, --single-branch)
- Improve error handling (%w wrapping) and logging consistency
- Add case-insensitive HTTP header lookup for webhooks

docker/Dockerfile

@@ -1,11 +1,12 @@
-FROM docker.io/golang:1.24.2-bookworm as build
+FROM docker.io/golang:1.25.5-bookworm as build
 WORKDIR /app
-# Copy dependencies list
-COPY ./app/go.mod ./
-COPY ./app/go.sum ./
+# NOTE: This Dockerfile assumes the build context is the repository root.
+# Example: docker build -f docker/Dockerfile .
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
 # Build with optional lambda.norpc tag
-COPY ./app/main.go ./
-RUN go build -tags lambda.norpc -o main main.go
+RUN go build -tags lambda.norpc -o main ./cmd/lambda
 # Copy artifacts to a clean image
 FROM public.ecr.aws/lambda/provided:al2023
 # Install git and zip using dnf (Amazon Linux 2023)