Add Helm chart for Kubernetes deployment

[mfournioux]

Summary

I've built a Helm chart to deploy nao on Kubernetes and would like to propose it as a PR (already opened, but auto-closed pending approval: #1100).

What's included

• Full chart structure: Deployment, Service, ConfigMap, Secret, ServiceAccount, PVC, HPA, PDB, and a helm test connection test
• Support for all three nao context modes via config.contextSource:
  • local — mount a pre-populated PVC containing the nao project
  • git — clone the project from a Git repo at pod startup, with optional scheduled refresh
• Optional bundled PostgreSQL via the bitnami/postgresql subchart, or connect to an external Postgres/SQLite instance via secrets.dbUri
• Configurable autoscaling (HPA) and PodDisruptionBudget
• Full values reference and installation/upgrade/rollback documentation

Installation example

```bash
helm repo add bitnami https://charts.bitnami.com/bitnami
helm dependency update ./helm
helm install nao ./helm
--namespace nao --create-namespace
--set secrets.betterAuthSecret="$(openssl rand -base64 32)"
```

Motivation

This makes it straightforward to run nao in production Kubernetes environments with proper scaling, persistence, and secret management, rather than requiring manual manifests.

Related PR: #1100

Happy to adjust based on maintainer feedback before opening/reopening the PR.

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