ci: add vault CLI to infra-ci image + surface real errors in scripts/tg

The Woodpecker CI pipeline has been silently failing to apply Tier 1
stacks since the state-migration commit e80b2f02 because the Alpine
CI image never had the vault CLI. `scripts/tg` swallowed stderr with
`2>/dev/null` and surfaced a misleading "Cannot read PG credentials
from Vault" message — the real error was `sh: vault: not found`.

Verified with an in-cluster probe: woodpecker/default SA + role=ci
already gets the terraform-state policy and has read capability on
database/static-creds/pg-terraform-state. Auth was never the problem;
the vault binary just wasn't there.

- ci/Dockerfile: pin vault v1.18.1 (matches server) and install
- scripts/tg: pre-flight check + surface real vault output on failure
- Next build-ci-image.yml run rebuilds :latest with vault included;
  subsequent default.yml runs unblock monitoring apply (code-aoxk)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ci/Dockerfile

@@ -1,12 +1,11 @@
 FROM alpine:3.20
 
-# Rebuild 2026-04-19 — previous :latest index referenced missing blobs (404 on 98f718c8 / 27d5ab83)
-
 # Pin versions to match CI requirements
 ARG TERRAFORM_VERSION=1.5.7
 ARG TERRAGRUNT_VERSION=0.99.4
 ARG SOPS_VERSION=3.9.4
 ARG KUBECTL_VERSION=1.34.0
+ARG VAULT_VERSION=1.18.1
 
 # Install system packages (single layer)
 RUN apk add --no-cache \
@@ -36,6 +35,16 @@ RUN curl -fsSL "https://dl.k8s.io/release/v${KUBECTL_VERSION}/bin/linux/amd64/ku
     -o /usr/local/bin/kubectl \
     && chmod +x /usr/local/bin/kubectl
 
+# Vault CLI — required by scripts/tg for Tier 1 stack PG credential reads
+# and Tier 0 advisory locks. Pinned to server version (1.18.1). Without this
+# the CI pipeline surfaces the misleading "Cannot read PG credentials" error
+# because scripts/tg swallows stderr ("vault: not found").
+RUN curl -fsSL "https://releases.hashicorp.com/vault/${VAULT_VERSION}/vault_${VAULT_VERSION}_linux_amd64.zip" \
+    -o /tmp/vault.zip \
+    && unzip /tmp/vault.zip -d /usr/local/bin/ \
+    && rm /tmp/vault.zip \
+    && vault version
+
 # Provider cache directory (shared across stacks)
 ENV TF_PLUGIN_CACHE_DIR=/tmp/terraform-plugin-cache
 ENV TF_PLUGIN_CACHE_MAY_BREAK_DEPENDENCY_LOCK_FILE=1