git-credential-mosaic: escape the escalation record, and stop naming a record that was never written

Both defects found in review by rev-code-01 on #1311.

F3 — the JSONL record interpolated every field with a bare %s. An identity comes
from git config or the environment and a cwd is whatever directory git ran in, so
either can contain a quote or a backslash. One such refusal turned the day's spool
into unparseable JSONL, and the operator would only discover it while reading the
record that explains an outage. Fields are now JSON-escaped.

F2 — the diagnostic printed "record: <spool>/<date>.jsonl" unconditionally, but
the record is only written inside the branch where mkdir -p succeeded. When the
spool cannot be created the helper named a file that does not exist, on exactly
the hosts where the escalation was lost. It now reports the real path or says
NOT WRITTEN.

Also: prettier on README.md, which was the format-step failure on pipeline 2508.
It reflowed only the two tables this branch added.

Tests: cases 14 and 15 cover both. Verified discriminating — against the previous
helper with these same tests, case 14 fails with the unparseable record printed
and case 15 fails on both assertions; against this one both pass.

The first draft of case 14 used `ls "$spool"/*.jsonl | head -1`, which under
`set -o pipefail` exits 2 on a missed glob and killed the suite with zero output
— the same silent-nonzero failure rev-code-01 hit from a partial tools/ extraction
and the reason this file exists. Replaced with a glob loop and a comment.

packages/mosaic/framework/tools/git/test-git-credential-mosaic.sh

@@ -290,13 +290,81 @@ assert_eq "unknown host on a fleet host: still passthrough, not a refusal" "" "$
 # ---------------------------------------------------------------------------
 store_out=$(cd "$REPO_DIR" && env -i HOME="$FAKE_HOME" PATH="$PATH" bash "$HELPER" store <<EOF
 host=git.mosaicstack.dev
-username=agentA
+username=no-such-agent
 password=whatever
 
 EOF
 )
 assert_eq "store verb: no output" "" "$store_out"
 
+# ---------------------------------------------------------------------------
+# 14. The escalation record is machine-readable even when a field carries a
+#     quote or a backslash. A cwd is arbitrary operator text; an unescaped one
+#     silently turns the spool into unparseable JSONL, and the operator only
+#     finds out while reading the record that explains an outage.
+# ---------------------------------------------------------------------------
+hostile_dir="$WORK_DIR/we\"ird\\dir"
+mkdir -p "$hostile_dir"
+hostile_spool="$WORK_DIR/spool-hostile"
+(
+  cd "$hostile_dir"
+  env -i HOME="$FAKE_HOME" PATH="$PATH" MOSAIC_CREDENTIAL_SPOOL="$hostile_spool" \
+    MOSAIC_GIT_IDENTITY=no-such-agent \
+    bash "$HELPER" get <<EOF >/dev/null 2>&1
+host=git.mosaicstack.dev
+username=no-such-agent
+
+EOF
+) || true
+# Deliberately not `ls ... | head -1`: under `set -o pipefail` a missed glob
+# makes ls exit 2, the pipeline inherits it, and `set -e` kills this suite with
+# zero output — the same silent-nonzero failure this file exists to catch.
+record_file=""
+for candidate in "$hostile_spool"/*.jsonl; do
+  if [[ -e "$candidate" ]]; then
+    record_file="$candidate"
+    break
+  fi
+done
+if [[ -z "$record_file" ]]; then
+  echo "FAIL: hostile cwd — no escalation record was written at all" >&2
+  fail=1
+elif ! python3 -c 'import json,sys
+for line in open(sys.argv[1]):
+    line = line.strip()
+    if line:
+        json.loads(line)' "$record_file" 2>/dev/null; then
+  echo "FAIL: hostile cwd — escalation record is not parseable JSONL:" >&2
+  cat "$record_file" >&2
+  fail=1
+fi
+
+# ---------------------------------------------------------------------------
+# 15. When the spool cannot be created, the diagnostic must NOT name a record
+#     path. Naming a file that was never written sends the operator to an
+#     empty path on exactly the hosts where the escalation was lost.
+# ---------------------------------------------------------------------------
+unwritable_spool="/proc/mosaic-credential-spool-cannot-exist"
+nospool_err=$(
+  cd "$REPO_DIR"
+  env -i HOME="$FAKE_HOME" PATH="$PATH" MOSAIC_CREDENTIAL_SPOOL="$unwritable_spool" \
+    MOSAIC_GIT_IDENTITY=no-such-agent \
+    bash "$HELPER" get <<EOF 2>&1 >/dev/null
+host=git.mosaicstack.dev
+username=no-such-agent
+
+EOF
+) || true
+if [[ "$nospool_err" == *"record: $unwritable_spool/"* ]]; then
+  echo "FAIL: unwritable spool — diagnostic names a record file that was never written" >&2
+  fail=1
+fi
+if [[ "$nospool_err" != *"NOT WRITTEN"* ]]; then
+  echo "FAIL: unwritable spool — diagnostic does not say the record was not written" >&2
+  echo "$nospool_err" >&2
+  fail=1
+fi
+
 if [[ "$fail" -eq 0 ]]; then
   echo "git-credential-mosaic identity resolution regression passed"
 fi