chore(ci): map chart updater app key for generated docs (#37)

## Summary

- map the repository `CHART_UPDATER_GITHUB_APP_PRIVATE_KEY` Actions
secret into the reusable OpenTofu workflow

## Context

Completes the chart updater App key migration for this root.
makeitworkcloud/shared-workflows#33 replaces the AWS OIDC + Secrets
Manager docs-commit credential with this secret;
makeitworkcloud/tfroot-github#45 provisions the secret to this
repository.

## Merge order

1. makeitworkcloud/shared-workflows#33
2. makeitworkcloud/tfroot-github#45 (apply provisions the secret)
3. This PR

CI on this PR calls the reusable workflow from `@main` and cannot pass
until shared-workflows#33 merges; re-run checks after that lands.

## Validation

- Reviewed the diff: one secret mapping added; no credentials or
decrypted values included.
- GitHub Actions checks are the validation authority after the
dependency merges.

Author: xnoto

.github/workflows/opentofu.yml

@@ -19,3 +19,5 @@ jobs:
       github.event_name != 'pull_request' ||
       github.event.pull_request.head.repo.full_name == github.repository
     uses: makeitworkcloud/shared-workflows/.github/workflows/opentofu.yml@main
+    secrets:
+      CHART_UPDATER_GITHUB_APP_PRIVATE_KEY: ${{ secrets.CHART_UPDATER_GITHUB_APP_PRIVATE_KEY }}