Remove SQLite and deploy Postgres by default

[iplay88keys]

Summary

Removes SQLite as a supported database backend and makes PostgreSQL the only supported option. Bundles a PostgreSQL instance (with pgvector) in the Helm chart so that helm install works out of the box with no external prerequisites.

Closes: #1502

Changes

Helm

• Bundle PostgreSQL in helm/kagent/templates/postgresql.yaml — deployed when database.postgres.url and database.postgres.urlFile are both empty (the default)
• Set database.postgres.url to skip the bundled instance and use an external PostgreSQL
• Remove database.type, database.sqlite, SQLite emptyDir volume, and XDG_CACHE_HOME from the controller deployment

CI

• Remove strategy.matrix: database: [sqlite, postgres] from test-e2e — single database, no matrix needed
• Remove separate Postgres service container — postgres is deployed inside the Kind cluster by make helm-install

Helm values

database:
  postgres:
    # Leave empty to deploy the bundled postgres. Set to use external PostgreSQL.
    url: ""
    urlFile: ""
    vectorEnabled: true
    bundled:
      image: pgvector/pgvector:pg18-trixie
      storage: 500Mi
      database: postgres
      user: postgres
      password: kagent

Test plan

• Go unit tests pass (go test -race -skip 'TestE2E.*' ./...)
• Helm unit tests pass (helm unittest helm/kagent)
• TestE2EMemoryWithAgent passes — confirms pgvector and memory table work end-to-end
• Full E2E suite passes (make helm-install push-test-agent push-test-skill followed by e2e tests via go test -v github.com/kagent-dev/kagent/go/core/test/e2e -failfast -shuffle=on)

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[dimetron]

Please consider support https://github.com/cloudnative-pg/cloudnative-pg
To have compatibility from the start and allow HA in production env.
@iplay88keys @EItanya

[iplay88keys]

Follow-up to this in: #1527