release: prepare v0.6.0

[vicotrbb]

Summary

• promote Devboxes to v0.6.0 after the tenant-scoped access control and multi-tenancy implementation
• synchronize release metadata across Helm, controller, CLI, installer, dashboard assets, tests, and documentation
• replace the Artifact Hub release placeholder with the v0.6.0 changelog
• retry Kubernetes resourceVersion conflicts during start and stop with bounded backoff and fresh tenant ownership validation

Validation

• scripts/check-version.sh
• make lint
• make test, 183 controller tests at 87.72% coverage and 28 CLI tests
• make helm, including lint, schema, rendered manifests, and chart packaging checks
• DEVBOXES_E2E_CLI=cli/target/debug/devbox scripts/kind-e2e.sh, fresh cluster including custom-image stop and start, role enforcement, and same-name tenant isolation
• post-merge CI for PR feat: add namespace-isolated multi-tenancy #46, all 9 required jobs passed on d61de7ae3f93147972a15c7e59a27aec38ecf9d8

Release implementation: #46
Release issue: #33