feat(helm): add Gateway Deployment + Service templates

[masami-agent]

Summary

Add Helm chart templates to deploy the OpenAB Gateway as part of helm install, so enterprise users don't need to manually create Gateway K8s resources.

Context

Currently the Helm chart only deploys OAB agent pods. The Gateway is deployed separately (BYO — see docs/msteams-enterprise.md). This works but requires users to manage two separate deployments.

Related PRs:

• refactor(gateway): split main.rs into adapter modules #550 — gateway adapter modular refactor (merged)
• feat(gateway): add MS Teams adapter + self-hosted docs #667 — Teams adapter + self-hosted docs (merged)
• docs(teams): add enterprise K8s deployment guide + Helm chart support #674 — enterprise deployment docs, BYO gateway (merged)

Scope

File	Description
charts/openab/templates/gateway.yaml	Gateway Deployment + Service (created when gateway.enabled=true)
charts/openab/templates/gateway-secret.yaml	Gateway-specific secrets (TEAMS_APP_SECRET, GATEWAY_WS_TOKEN)
charts/openab/values.yaml	Add gateway.image, gateway.tag, inject TEAMS_* env vars into gateway container

Expected Behavior

helm install openab oci://ghcr.io/openabdev/charts/openab \
  --set agents.kiro.gateway.enabled=true \
  --set agents.kiro.gateway.teams.appId="..." \
  --set-literal agents.kiro.gateway.teams.appSecret="..."

Creates:

• OAB Deployment (agent container)
• Gateway Deployment (gateway container with TEAMS_* env vars)
• Gateway Service (ClusterIP :8080)
• Gateway Secret (TEAMS_APP_SECRET)

Notes

• Gateway image: ghcr.io/openabdev/openab-gateway
• TEAMS_* env vars must be injected into the gateway container, not the agent container
• Condition for Teams env injection: appId AND appSecret both set (fail-closed)

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