feat: CI/CD performance overhaul

- New custom CI Docker image (ci/Dockerfile) with TF 1.5.7, TG 0.99.4,
  git-crypt, sops, kubectl pre-installed. Pushed to private registry.
  Eliminates 17 apk add calls + binary downloads per pipeline run.

- Unified CI pipeline: merge default.yml + app-stacks.yml into one.
  Changed-stacks-only detection (git diff, with global-file fallback).
  Concurrency limit (xargs -P 4). Step consolidation (2 steps vs 4).
  Shallow clone (depth=2). Provider cache (TF_PLUGIN_CACHE_DIR).

- Per-stack Vault advisory locks in scripts/tg. 30min TTL with stale
  lock detection. Blocks concurrent applies to same stack.

- TF_PLUGIN_CACHE_DIR enabled by default in scripts/tg for local dev.

- Daily drift detection pipeline (.woodpecker/drift-detection.yml).
  Runs terraform plan on all stacks, Slack alert on drift.

- CI image build pipeline (.woodpecker/build-ci-image.yml).

Expected speedup: ~5-10 min per pipeline run → ~2-4 min.

[ci skip]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ci/Dockerfile

@@ -0,0 +1,42 @@
+FROM alpine:3.20
+
+# 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
+
+# Install system packages (single layer)
+RUN apk add --no-cache \
+    bash curl git git-crypt jq openssh-client openssl unzip \
+    && rm -rf /var/cache/apk/*
+
+# Terraform
+RUN curl -fsSL "https://releases.hashicorp.com/terraform/${TERRAFORM_VERSION}/terraform_${TERRAFORM_VERSION}_linux_amd64.zip" \
+    -o /tmp/terraform.zip \
+    && unzip /tmp/terraform.zip -d /usr/local/bin/ \
+    && rm /tmp/terraform.zip \
+    && terraform version
+
+# Terragrunt
+RUN curl -fsSL "https://github.com/gruntwork-io/terragrunt/releases/download/v${TERRAGRUNT_VERSION}/terragrunt_linux_amd64" \
+    -o /usr/local/bin/terragrunt \
+    && chmod +x /usr/local/bin/terragrunt \
+    && terragrunt --version
+
+# SOPS (for state encryption)
+RUN curl -fsSL "https://github.com/getsops/sops/releases/download/v${SOPS_VERSION}/sops-v${SOPS_VERSION}.linux.amd64" \
+    -o /usr/local/bin/sops \
+    && chmod +x /usr/local/bin/sops
+
+# kubectl
+RUN curl -fsSL "https://dl.k8s.io/release/v${KUBECTL_VERSION}/bin/linux/amd64/kubectl" \
+    -o /usr/local/bin/kubectl \
+    && chmod +x /usr/local/bin/kubectl
+
+# 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
+RUN mkdir -p /tmp/terraform-plugin-cache
+
+WORKDIR /workspace