fix(git): #1007 — test-issue-comment-readback is a FIFTH affected suite (second census correction)

My previous commit said four. It is five. `test-issue-comment-readback.sh` has
the same defect and is fixed the same way, and I had already looked straight at
it and filed it as an *unrelated* silent failure. Correcting that here rather
than folding it in quietly.

WHY IT WAS MISSED — the general lesson, not the excuse. `run_comment()` sends
the wrapper's stdout AND stderr to `$OUTPUT_FILE`, and the `EXIT` trap deletes
`$WORK_DIR`. The suite therefore exits 1 with ZERO bytes on stdout and stderr,
and the one line that says what went wrong —

    Error: Gitea authenticated-identity read failed with HTTP 401

— lives only inside a directory that no longer exists when anyone looks. Every
oracle I had swept the family with greps for a SYMPTOM in surviving output, so
against this suite all of them returned "nothing found", which I read as "clean"
in the first sweep and as "unrelated pre-existing failure" in the second. A
suite that discards or deletes its own evidence converts a post-hoc assay into a
non-measurement, and I wrote that sentence into the previous commit while it was
already false about a file in the same directory.

HOW IT WAS ACTUALLY FOUND. Intercept the identity read at its SOURCE instead of
grepping for its consequence: a PATH shim over `git` that logs every
`mosaic.gitIdentity` read — args, rc, and resolved value — to a file OUTSIDE any
suite's work dir, then execs the real git. Deletion-proof by construction, and
it measures the defect's cause rather than one of its symptoms. Sweeping all 16
suites with it under an ordinary invocation:

  resolves a REAL identity (`mos-dt-0`) before the fix:
    test-issue-comment-readback          1 read   rc=1 (RED on every seat)
    test-pr-review-repo-host-override    6 reads  rc=0
    test-ci-queue-wait-branch-absent     3 reads  rc=0
  the four fixed in the previous commit now read empty; the rest never read at all.

The latter two are NOT affected and are deliberately left alone: under a seat
replica (identity set, no per-slot token) neither reaches `get_gitea_token`'s
fail-loud branch, and under a canary HOME neither carries the canary credential
into any surviving artifact. They read the identity and never enter a credential
path. That residual is structural and belongs to the wrapper half of #1007 —
scoping the read with `git -C "$repo"` removes it for everyone at once.

An earlier version of that sweep reported the four fixed suites as still
resolving a real identity. That was my grep, not the suites: `value=\[..*\]` is
satisfied by `value=[] args=[…]`, because `.*` runs past the empty pair and
matches the closing bracket of the NEXT one. `value=\[[^]]` is the correct test.
Recorded because the wrong pattern failed in the direction that would have sent
me re-fixing four already-correct files.

VERIFICATION of this suite, four HOME arms, all rc=0 with zero non-empty
identity reads and the pass line on stdout: real HOME, seat replica, canary
HOME, and an empty HOME with no identity at all. Full 16-suite sweep after the
change: every suite rc=0.

CONSEQUENCE FOR THE FINDING LIST IN THE PREVIOUS COMMIT: item 2 there — the
"silently red, unrelated to #1007" suite — is withdrawn. It was #1007 all along.
Item 1 (`pr-metadata.sh:89-92`, the anonymous fallback that reports an HTTP 200
carrying valid JSON as "unknown API error") stands and is still unfixed here.

Refs #1007

packages/mosaic/framework/tools/git/test-issue-comment-readback.sh

@@ -61,15 +61,54 @@ STATE_FILE="$WORK_DIR/comments.json"
 # A dedicated scratch dir the wrapper is pointed at via TMPDIR, so the leak
 # check can assert every POST/GET body + metadata temp file is cleaned up.
 TMP_SCRATCH="$WORK_DIR/scratch"
+HOME_DIR="$WORK_DIR/home"
 
 cleanup() {
     rm -rf "$WORK_DIR"
 }
 trap cleanup EXIT
 
-mkdir -p "$REPO_DIR" "$BIN_DIR" "$XDG_DIR" "$TMP_SCRATCH"
+mkdir -p "$REPO_DIR" "$BIN_DIR" "$XDG_DIR" "$TMP_SCRATCH" "$HOME_DIR"
 git -C "$REPO_DIR" init -q
 git -C "$REPO_DIR" remote add origin https://git.mosaicstack.dev/mosaicstack/stack.git
+# HERMETICITY (#1007) — TWO mechanisms with DIFFERENT jobs; do not conflate them.
+#
+# OPERATIVE: the empty repo-local `mosaic.gitIdentity` below. get_gitea_token()
+# step 0 resolves a per-agent identity from `git config --get mosaic.gitIdentity`,
+# which on a provisioned agent seat is set GLOBALLY and so leaks into this fresh
+# repo. It then reads a REAL per-slot token from $HOME and returns it WITHOUT ever
+# consulting MOSAIC_CREDENTIALS_FILE, so the fixture credential below is silently
+# ignored. The stub curl then rejects the unrecognised bearer, and this suite
+# fails at its FIRST case with `Gitea authenticated-identity read failed with
+# HTTP 401`. An empty repo-local value shadows the global one and reads back
+# empty at rc=0. Measured: without this pin the suite is RED on every seat.
+#
+# CONTAINMENT: the sandboxed HOME in run_comment(). It only has to bound a
+# failure that the pin should already have prevented.
+#
+# THIS SUITE WAS THE HARDEST OF THE FIVE TO SEE, and the reason is worth stating
+# because it generalises: run_comment() sends the wrapper's stdout AND stderr to
+# $OUTPUT_FILE, and the EXIT trap above deletes $WORK_DIR. So the 401 — the only
+# thing that says what went wrong — exists only inside a directory that is gone
+# by the time anyone looks. The suite exits 1 with ZERO bytes on stdout and
+# stderr. A suite that discards or deletes its own evidence turns any post-hoc
+# assay into a non-measurement: "nothing found" there means "no surviving
+# trace", never "clean". It was found by intercepting the identity read at its
+# SOURCE (a PATH shim over `git` logging every `mosaic.gitIdentity` read to a
+# file outside $WORK_DIR), which is deletion-proof by construction, rather than
+# by grepping for the symptom.
+#
+# NOTE FOR ANYONE AUDITING THIS SUITE: the sandboxed HOME is containment, NOT an
+# assay. Running a suite under a decoy HOME to test for this defect REMOVES the
+# trigger — ~/.gitconfig is where the global identity lives, so step 0 is skipped
+# by construction and every suite reads clean however vulnerable it is. To
+# measure, REPLICATE a seat (a decoy HOME whose .gitconfig sets
+# mosaic.gitIdentity, with no per-slot token) so step 0 reaches its fail-loud
+# branch — or intercept the read as described above.
+#
+# Note the env-var route does NOT work: detect-platform.sh reads
+# "${MOSAIC_GIT_IDENTITY:-}", and `:-` treats set-but-empty identically to unset.
+git -C "$REPO_DIR" config mosaic.gitIdentity ""
 
 ISSUE_NUMBER=7
 REPO_SLUG="mosaicstack/stack"
@@ -366,6 +405,7 @@ run_comment() {
         cd "$REPO_DIR"
         PATH="$BIN_DIR:$PATH" \
         TMPDIR="$TMP_SCRATCH" \
+        HOME="$HOME_DIR" \
         XDG_CONFIG_HOME="$XDG_DIR" \
         MOSAIC_CREDENTIALS_FILE="$CREDENTIALS_FILE" \
         ISSUE_COMMENT_TEA_LOG="$TEA_LOG" \