fix: replace client-go informer with REST loader, drop k8s deps

Informer cache sync timed out due to transient 429 from API server.
REST loader does one GET + retry, polls every 30s. Zero new deps.
go.mod back to 1.25.4, CI back to golang:1.25-bookworm.

Dockerfile

@@ -7,7 +7,7 @@
 # --platform=$BUILDPLATFORM pins the build stage to the machine doing the
 # building, then Go cross-compiles to $TARGETARCH. Without it, building an
 # amd64 image from an arm64 workstation runs the whole toolchain under QEMU.
-FROM --platform=$BUILDPLATFORM golang:1.26-bookworm AS build
+FROM --platform=$BUILDPLATFORM golang:1.25-bookworm AS build
 
 WORKDIR /src