feat: add kplane storage implementation

[savme]

Related to #596.
Alternative implementation in #647.

What

Adds a kplane storage implementation to Milo.

Numbers

Scale benchmark; 100 projects across 5 runs (using the harness from #615):

metric	value per project
heap	3.0 MiB
sys	5.0 MiB
goroutines	90
etcd watchers	0

How to try it

task dev:setup:kplane                 
task test:scale                       # run the scale benchmark (default 20 projects)
SCALE_PROJECTS=100 task test:scale    # or using a specific number of projects to create

Notes

Toolchain bumps required by the kplane fork:

• Go: 1.25 → 1.26.3
• sigs.k8s.io/controller-runtime: 0.23.3 → 0.24.1
• sigs.k8s.io/multicluster-runtime: 0.23.3 → 0.24.1
• k8s.io/{api,apimachinery,apiserver,client-go,component-base,...}: v0.35.0 → v0.36.0

k8s.io/client-go v1.5.2 appears in go.mod per kplane's require https://github.com/kplane-dev/apiserver/blob/main/go.mod#L17

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./... (unit) clean
• task dev:setup:kplane brings apiserver up with the flag (verified via the kplane shared-cacher log line)
• task test:scale against kplane backend produces the numbers above
• Chainsaw e2e suite