feat: add LangGraph BYO agent example

[hsteude]

Summary

• add a deterministic LangGraph website-research workflow exposed through kagent A2A
• package it as a hardened non-root BYO agent image with Anthropic secret injection
• add an Artifact Registry build workflow and standalone usage documentation
• stack this example on feat: add progressive and LangGraph BYO kagent examples #62

Testing

• pytest: 5 passed in the built linux/amd64 image
• black --check and isort --check-only
• yamllint for the workflow and Agent manifest
• built and started the linux/amd64 image; /health and the A2A agent card passed
• fetched https://prokube.ai through the deterministic workflow node
• pushed latest and commit-064d18982baf85544456619742b48a721b295b31 with digest sha256:51354f341a383d747565f27e176ed92775c6274a8773c6b98ed446040086030a

Not Run

• cluster deployment and end-to-end Anthropic invocation because the test cluster was intentionally shut down