Add optional example agent to Helm chart

[tomkis]

Include an example agent as part of the Helm chart, deployable at startup. This replaces the old registry-based approach — since the registry was removed, Kagenti now manages deployed resources directly via Kubernetes.

Requirements

• Add a Helm value exampleAgent (default: false)
• When true, deploy an example agent as part of the chart
• adk platform start sets exampleAgent=true by default
• Can be disabled via adk platform start --set=exampleAgent=false (or similar)

Context

• Related PR: feat: adk support agent #173
• The agent should not use the BeeAI framework (to avoid a2a issues)