Build custom Kingfisher container from sporked deploy branch (#318)

## Summary

- Add Dockerfile for Kingfisher built from source (sporked deploy branch)
- Multi-stage: Rust build with Boost/vectorscan, debian-slim runtime
- Switch CronJob from upstream `ghcr.io/mongodb/kingfisher` to `registry.ops.eblu.me/blumeops/kingfisher`
- Add kingfisher to service-versions.yaml (version tracks upstream main SHA)
- Document spork workflow in CLAUDE.md

## Test plan

- [ ] Build container: `mise run container-build-and-release kingfisher 1d37d29`
- [ ] Verify image on registry: `mise run container-list`
- [ ] Update kustomization newTag
- [ ] Sync ArgoCD kingfisher app from branch
- [ ] Trigger manual CronJob and verify scan completes
- [ ] Verify reports on sifaka

Reviewed-on: #318

docs/explanation/spork-strategy.md

@@ -60,6 +60,7 @@ Note that a cron-triggered workflow is especially dangerous: it requires no user
 
 - [[create-a-spork]] — initial setup with `mise run spork-create`
 - [[manage-spork-branches]] — feature branches, the deploy branch, handling rebase conflicts
+- [[build-spork-container]] — building reproducible containers from pinned SHAs
 
 ## See also