fix(ci): free-tests container parity — tools, pinned bun, git identity, mutation tripwire

- Dockerfile.ci: add python3 (gstack-jsonl-merge/brain-sync/detach shell out
  to it), file (skill-validation's binary check), poppler-utils (make-pdf
  e2e gates hard-require pdftotext/pdffonts/pdfinfo), fonts-noto-color-emoji
  (emoji render gate, mirrors make-pdf-gate.yml). Fix the bun pin: the
  bun.sh installer ignores a BUN_VERSION env var, so the old form silently
  installed latest on every rebuild (observed 1.3.13/1.3.14 drift vs the
  1.3.10 devs run locally); pass the version as the positional arg.
- free-tests.yml: git identity + safe.directory for the git-exercising
  tests (container checkout is owned by a different uid than runner);
  post-loop tree-mutation tripwire that names a tracked-file-mutating test
  instead of letting downstream collateral confuse the report; skip the
  documented variants-retry-after timing flake.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/docker/Dockerfile.ci

@@ -28,9 +28,13 @@ RUN printf 'Acquire::Retries "5";\nAcquire::http::Timeout "30";\nAcquire::https:
 
 # System deps (retry apt-get update + install as a unit — even Hetzner can blip).
 # Includes xz-utils so the Node.js .tar.xz download below can decompress.
+# python3: bin/gstack-jsonl-merge, gstack-brain-sync, gstack-detach, and other
+#   bash bins shell out to it (macOS ships python3; the base image doesn't).
+# file: skill-validation's no-compiled-binaries-in-git check runs `file --mime-type`.
+# poppler-utils: make-pdf's e2e gates hard-require pdftotext/pdffonts/pdfinfo in CI.
 RUN for i in 1 2 3; do \
       apt-get update && apt-get install -y --no-install-recommends \
-        git curl unzip xz-utils ca-certificates jq bc gpg && break || \
+        git curl unzip xz-utils ca-certificates jq bc gpg python3 file poppler-utils && break || \
       (echo "apt retry $i/3 after failure"; sleep 10); \
     done \
     && rm -rf /var/lib/apt/lists/*
@@ -61,10 +65,14 @@ RUN curl --retry 5 --retry-delay 5 --retry-connrefused -fsSL "https://nodejs.org
     && node --version \
     && npm --version
 
-# Bun (install to /usr/local so non-root users can access it)
+# Bun (install to /usr/local so non-root users can access it).
+# The version MUST be passed as a positional arg — bun.sh/install ignores a
+# BUN_VERSION env var, so the old `| BUN_VERSION=x.y.z bash` form silently
+# installed latest on every image rebuild (observed: 1.3.13/1.3.14 drift vs
+# the 1.3.10 devs run locally).
 ENV BUN_INSTALL="/usr/local"
 RUN curl --retry 5 --retry-delay 5 --retry-connrefused -fsSL https://bun.sh/install \
-    | BUN_VERSION=1.3.10 bash
+    | bash -s "bun-v1.3.10"
 
 # Claude CLI
 RUN npm i -g @anthropic-ai/claude-code
@@ -82,8 +90,10 @@ RUN npx playwright install-deps chromium
 # (headed-xvfb, headed-orphan-cleanup) can exercise the Linux container
 # auto-spawn path on every CI run. Without Xvfb in the image, the most
 # common production --headed path goes untested.
+# fonts-noto-color-emoji: the make-pdf emoji render gate needs a color-emoji
+# fallback font (mirrors make-pdf-gate.yml's Ubuntu setup step).
 RUN for i in 1 2 3; do \
-      apt-get update && apt-get install -y --no-install-recommends fonts-liberation fontconfig xvfb x11-utils && break || \
+      apt-get update && apt-get install -y --no-install-recommends fonts-liberation fonts-noto-color-emoji fontconfig xvfb x11-utils && break || \
       (echo "fonts-liberation install retry $i/3"; sleep 10); \
     done \
     && fc-cache -f \
@@ -105,6 +115,7 @@ RUN npx playwright install chromium \
 
 # Verify everything works
 RUN bun --version && node --version && claude --version && jq --version && gh --version \
+    && python3 --version && command -v file && command -v pdftotext && command -v pdffonts && command -v pdfinfo \
     && npx playwright --version \
     && fc-match "Liberation Sans" | grep -qi "Liberation" \
         || (echo "ERROR: fonts-liberation not installed — make-pdf PDFs will render in DejaVu Sans" && exit 1)