feat: shared cluster in previews-shared scopes secrets per env

[BSick7]

Lets one cluster serve every Nullstone preview env while keeping their secrets apart. Nothing changes for a cluster that is not shared.

Shared mode

The cluster decides: aws-ec2-vault-cluster sets shared = data.ns_env.this.type == "PreviewsSharedEnv", exposes it as output shared, and writes SHARED_ENVS into user-data. Everything else reads that one value. When true:

• Tenant roles are named <env>.<tenant> on the same two AppRole mounts, each bound to an identity entity carrying env and tenant metadata. Provisioning creates the entity and its alias on the role's role_id.
• Tenant policies are scoped to kv/<family>/envs/{{entity.env}}/customers/{{entity.tenant}}/* with the whole envs/ prefix denied otherwise.
• Broker policies are scoped to auth/approle-<kind>/role/{{entity.env}}.* with every parameter denied: an app mints logins only for its own env and cannot change a role. The env comes from the entity the cluster function writes on the app role (feat: bind app roles to an env-scoped entity on shared clusters vault-admin#6).
• Provisioning and apps-auth get narrow identity grants with policies and disabled refused; groups, OIDC, MFA, and merge stay denied.

Unshared clusters render the 0.1.x policies byte for byte. Goldens under testdata/policies/unshared pin that; testdata/policies/shared documents the shared output.

Modules

• aws-ec2-vault-cluster 0.2.0: output shared, SHARED_ENVS in user-data, pins vault-admin ~> 0.3.0. tofu init on this module fails until vault-admin v0.3.0 is published; the tests here ran against 0.2.2 with the pin swapped locally.
• aws-vault-access 0.3.0: reads the cluster's shared output (try(..., false), so 0.1.x clusters read as unshared), passes env to the function only when shared, injects VAULT_ENV (always present, empty when unshared).
• READMEs for both modules. Cluster: where each tenant's secrets live, how tenant tokens are scoped, what apps and humans can do. Access: the two-step login, token lifetimes, gotchas. .gitignore now keeps aws/*/README.md.

CLI

• tenants create|destroy take --env, required when shared and refused otherwise.
• tenants list shows the env and flags a role that does not fit the cluster mode.
• New envs list and envs destroy <env> --yes [--purge-secrets].
• vault-utils env prints SHARED_ENVS=true for a shared cluster.

Tests

• Existing Docker tests run unchanged with SharedEnvs=false.
• New shared tests: same tenant ID in two envs cannot cross, parents and wildcards and traversal denied, broker confined to its env and unable to rewrite a role, provisioning cannot grant entity policies, env destroy with purge leaves the other env intact, a login before the entity existed is relinked.
• tofu test in both modules covers shared, unshared, and a cluster with no shared output.
• A spike against Vault proved the templated glob, missing-metadata, and denied_parameters behaviors before the design was committed.

Release order

1. Tag and publish vault-admin v0.3.0 (feat: bind app roles to an env-scoped entity on shared clusters vault-admin#6).
2. Merge this. Bake the node AMI. Publish aws-ec2-vault-cluster 0.2.0 with --include='templates/*' and aws-vault-access 0.3.0.