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

[xnoto]

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. feat(opentofu): source docs GitHub App key from Actions secret shared-workflows#33
2. feat(actions): distribute chart updater app key to OpenTofu roots 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.

[github-actions]

OpenTofu CI passed

View the workflow run.

Validation output (redacted, last 12000 characters)

Terraform validate.......................................................Passed
Terraform validate with tflint...........................................Passed
Checkov..................................................................Passed
Terraform fmt............................................................Passed
Terraform docs...........................................................Passed
Detect hardcoded secrets.................................................Passed
check for case conflicts.................................................Passed
check for merge conflicts................................................Passed
check for broken symlinks............................(no files to check)Skipped
check vcs permalinks.....................................................Passed
detect destroyed symlinks................................................Passed
detect private key.......................................................Passed
fix end of files.........................................................Passed
mixed line ending........................................................Passed
trim trailing whitespace.................................................Passed
don't commit to branch..................................................Skipped
check for added large files..............................................Passed

[github-actions]

OpenTofu Plan