docs: add Helm to CONTRIBUTING prerequisites for chart tests

CONTRIBUTING.md

@@ -144,6 +144,14 @@ honest about being one.
 
 - [Bun](https://bun.sh/) (recommended) or Node.js 24+
 - Git
+- [Helm](https://helm.sh/) 4.1.3 (the version CI runs). Ten of the eleven `helm-chart-*.test.ts` files under `tests/unit/` spawn the `helm` binary - all but `helm-chart-readme-recipes.test.ts`, which is a static lint over the chart README. Without `helm` on `PATH`, `bun run test` fails with 166 `error: Executable not found in $PATH: "helm"` errors. The PostgreSQL subchart tarball is gitignored (`*.tgz`), so a fresh clone also needs:
+
+  ```bash
+  helm repo add bitnami https://charts.bitnami.com/bitnami
+  helm dependency build charts/libredb-studio --skip-refresh
+  ```
+
+  Trap: a stale `docker login` can make that build fail with `401 Unauthorized` from `registry-1.docker.io` even though the chart is anonymously pullable. `docker logout` fixes it.
 
 ### Getting Started
 
@@ -229,7 +237,8 @@ bun run format           # Biome formatter check (format:fix to write)
 bun run lint             # oxlint, then ESLint 9
 bun run typecheck        # TypeScript strict
 bun run knip             # unused files, exports and dependencies
-bun run test             # every test layer; never bare `bun test`
+bun run test             # every test layer; never bare `bun test`. Needs Helm and the built subchart, see Prerequisites
+bun run test:ci          # the same layers with one process per file, which is what CI runs; use it to verify
 bun run test:coverage    # coverage report (merged lcov)
 bun run coverage:check   # enforce 100% line coverage on the merged lcov
 bun run readme:check     # localized README drift guard