Bundle A: Container & supply-chain hardening — 3 findings closed; All High closed

Closes H-001 + M-012 + M-014 from comprehensive-audit-2026-04-25.

H-001 (CWE-829) — Container base images SHA-pinned
  Pre-bundle: 5 FROM lines pulled by tag only — registry-side tag
  swap could silently change the build.
  Post-bundle: every FROM pinned to immutable digest fetched live
  from Docker Hub at audit time:
    node:20-alpine@sha256:fb4cd12c85ee03686f6af5362a0b0d56d50c58a04632e6c0fb8363f609372293
    golang:1.25-alpine@sha256:5caaf1cca9dc351e13deafbc3879fd4754801acba8653fa9540cea125d01a71f (x2)
    alpine:3.19@sha256:6baf43584bcb78f2e5847d1de515f23499913ac9f12bdf834811a3145eb11ca1 (x2)
  Dockerfile header comment documents the operator bump procedure
  (quarterly cadence; docker manifest inspect or Hub Registry API).
  CI step Forbidden bare FROM regression guard (H-001) fails build
  if any new FROM lacks @sha256.

M-012 (CWE-250) — Verified-already-clean + USER guard
  Recon found both Dockerfile:75 and Dockerfile.agent:59 already
  carry USER certctl directives; pre-USER RUN calls are build-setup
  steps that legitimately need root, each happening before the
  USER drop.
  CI step Forbidden missing USER regression guard (M-012) greps
  every Dockerfile* for the LAST USER directive; fails build if
  missing OR equals root/0. Future Dockerfile additions must
  preserve the privilege drop.

M-014 — npm ci explicit retry helper
  Pre-bundle Dockerfile:25:
    RUN npm ci --include=dev || npm ci --include=dev && \
        tsc --version && npm run build
  Broken bash precedence: A || (B && C && D) means tsc+build only
  ran on success path of the second npm ci. A transient registry
  blip silently skipped the production step — build would succeed
  with no node_modules + no tsc verification.
  Post-bundle: deterministic 3-attempt retry loop with 5s backoff
  plus explicit [ -d node_modules ] post-check that fails loudly
  if directory wasn't created. Silent failure is now impossible.

Audit deliverables:
  audit-report.md: H-001/M-012/M-014 flipped [x] with closure
    notes; score 49/55 closed (High 9/9 = 100%; Medium 24/27;
    Low 19/19 with L-004 deferred). All High audit findings now
    closed for the first time.
  findings.yaml: 3 status flips
  CHANGELOG.md: Bundle A section

Verification:
  Self-test of both new CI guards locally — PASS for current state
  (every FROM has @sha256; every Dockerfile drops to non-root).

Dockerfile.agent

@@ -1,6 +1,11 @@
 # Multi-stage build for certctl agent
+#
+# Bundle A / Audit H-001 (CWE-829): every FROM line is pinned to an
+# immutable digest. See Dockerfile (server) for the bump-procedure
+# operator runbook; the pins here MUST be bumped in the same pass.
+
 # Stage 1: Build
-FROM golang:1.25-alpine AS builder
+FROM golang:1.25-alpine@sha256:5caaf1cca9dc351e13deafbc3879fd4754801acba8653fa9540cea125d01a71f AS builder
 
 # Proxy propagation (M-4, Issue #9) — defaulted to empty so un-proxied builds
 # behave identically to the pre-fix tree. When `HTTP_PROXY`/`HTTPS_PROXY`/
@@ -34,7 +39,7 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} go build \
     ./cmd/agent
 
 # Stage 2: Runtime
-FROM alpine:3.19
+FROM alpine:3.19@sha256:6baf43584bcb78f2e5847d1de515f23499913ac9f12bdf834811a3145eb11ca1
 
 # U-2: `procps` ships pgrep, which the HEALTHCHECK below uses to verify the
 # agent process is alive. Pre-U-2 the deploy/docker-compose.yml agent