fix(examples): build the docker fleets on Go 1.26 (#244)

The dependency bump raised go.mod to go 1.26.0, but the fleet and gateway
integration images still built on golang:1.25-alpine with
GOTOOLCHAIN=local, so `go mod download` refused to run and both
integration jobs failed.


Claude-Session: https://claude.ai/code/session_01Y5rfBgKpH7D2y2eRRVLsCq

Co-authored-by: Claude <noreply@anthropic.com>

Author: outofcoffee

examples/fleet-docker/Dockerfile

@@ -6,7 +6,7 @@
 
 # Build spinloop from the working tree, so the stack tests THIS commit rather
 # than a published artifact.
-FROM golang:1.25-alpine AS build
+FROM golang:1.26-alpine AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download

examples/gateway-docker/Dockerfile

@@ -7,7 +7,7 @@
 
 # Build spinloop from the working tree, so the stack tests THIS commit rather
 # than a published artifact.
-FROM golang:1.25-alpine AS build
+FROM golang:1.26-alpine AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download