feat: add HashiCorp Vault parameter provider

[fedemaleh]

Summary

Adds the HashiCorp Vault parameter provider (store, retrieve, delete), tested and ready to merge.

Split out from feature/azure-keyvault-hashicorp-vault-providers; the Azure Key Vault provider ships separately in its own PR.

What's included

• Provider scripts: parameters/providers/hashicorp-vault/{setup,store,retrieve,delete}
• Install specs (specs/install/) building on the shared parameter_storage_definition / parameter_storage_configuration modules, plus per-instance config and opt-in agent notification channel
• Kubernetes auth requirements (specs/requirements/): token-reviewer SA + cluster role binding, agent SA
• userpass and kubernetes authentication modes
• Shared install lock file (parameters/providers/shared/install/.terraform.lock.hcl)
• Docs: README.md, docs/architecture.md
• BATS tests under parameters/tests/providers/hashicorp-vault/
• CHANGELOG entry

Test plan

• BATS unit tests pass (setup, store, retrieve, delete)
• Manually validated store/retrieve/delete against a Vault instance