* feat: add OCI Helm registry support for agent deployments

Add support for deploying agents using OCI-based Helm registries (e.g., Google Artifact Registry) as an alternative to classic Helm repositories.

Changes:
- Add `helm_oci_registry` and `helm_chart_version` fields to AgentEnvironmentConfig
- Implement auto-login to Google Artifact Registry using gcloud credentials
- Add `--use-latest-chart` CLI flag to fetch the latest chart version from OCI registry
- Support both classic Helm repo mode and OCI registry mode based on environment config

* .

* .

* refactor: nest OCI fields into sub-model and extract resolve_chart helper

Address PR #255 review comments:
- Create OciRegistryConfig sub-model replacing flat helm_oci_* fields
- Remove use_oci boolean, branch on oci_registry presence directly
- Extract resolve_chart() combining chart reference + version resolution
- Add tests for nested OCI config YAML parsing

Replace direct Postgres checkpointing with HTTP-proxied checkpoint
operations through the agentex backend API. Agents no longer need
DATABASE_URL or direct DB connections for LangGraph state persistence.

- Add HttpCheckpointSaver that proxies through AsyncAgentex client
- Add create_checkpointer() factory using the HTTP checkpointer
- Replace langgraph-checkpoint-postgres dep with langgraph-checkpoint
- Export checkpointer module from adk package

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

feat: HTTP-proxy LangGraph checkpointer