The SOPS install + version, the decrypt loop, the OpenTofu version, and
the Backblaze backend-credential extraction were copy-pasted across
terraform.yml (twice), validate-terraform.yml, and _deploy-core.yml.
A version bump meant editing the same string in up to four places and
was easy to do partially.
Pull them into three local composite actions so each is defined once:
- setup-tofu (pins OpenTofu version)
- sops-decrypt (installs SOPS, decrypts *.enc.* in place)
- tofu-backend-creds (exports Backblaze S3 creds to GITHUB_ENV)
Behaviour is unchanged; sops-decrypt also matches *.enc.env everywhere
(previously only _deploy-core did), which is a no-op in terraform/.
