Wire dynamic warm capacity reconciliation

[bill-ph]

Summary

• replace the feat(controlplane): unbounded K8s worker scaling — remove memory-budget-derived MaxWorkers cap #597 per-control-plane warmCapacityMisses counter with configstore-backed dynamic warm-capacity reconciliation
• have the janitor compute base image targets from the latest snapshot, read recent no-idle miss buckets, compute effective image targets, and apply them with SpawnMinWorkersForImage
• keep dynamic warm capacity enabled by default so the configstore path preserves the existing burst-absorption behavior
• expose the dynamic warm-capacity knobs through CLI flags, root usage text, README config docs, and duckgres.example.yaml
• replace the old local-counter scaling test with tests for bucket-derived target computation and image-keyed spawning

Tests

• go test ./configresolve
• go test ./controlplane
• go test ./cmd/duckgres-controlplane
• go test .
• go test -tags kubernetes ./controlplane
• go test -tags kubernetes ./controlplane ./cmd/duckgres-controlplane
• go test -run 'TestListWarmCapacityMissesSinceAggregatesByScopeAndReasonPostgres|TestPruneWarmCapacityMissBucketsPostgres|TestRecordWarmCapacityMiss' ./tests/configstore

Review

• Final review found no P1/P2 issues.
• Addressed review findings by making dynamic warm capacity default-on and adding the new K8s CLI flags to README.