framework tools/git: sync issue-comment R1/R4 + hermeticity hardening from brain

Upstream of brain commits 08a00149 + 971586ef + the arm-6 sandbox fix
(brain 5th commit of 2026-08-28 series):

- R1: -b/--body is the canonical comment flag (matches issue-create,
  issue-edit, pr-create, pr-edit); -c/--comment stays a compatible alias.
- R4: usage errors print to stderr and exit 2, distinct from provider,
  credential, and verification failures (exit 1). Value-less flags fail
  loudly (previously -c with no value died silently at rc=1 via set -e on
  the failed shift 2). gh failures on the GitHub path normalize to exit 1
  (gh's own usage errors exit 2 and would collide with the reserved code).
- Tests: new usage-contract suite (help rc, unknown/missing/value-less rc=2
  on stderr, alias parse acceptance under a sandboxed runner, GitHub-path
  exit normalization with a stubbed gh, zero provider contact on parser
  failure); readback suite gains case 11 (full verified write via -b) and
  neutralizes seat-exported MOSAIC_GIT_IDENTITY / MOSAIC_BRAIN_HOME that
  escape the sandboxed HOME (documented HTTP 401 / fail-loud shapes).

Driver: a fleet seat full-stopped on an issue-comment usage error because
usage failures were indistinguishable from provider failures and the stop
gate treated every wrapper failure as blocking.

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

@@ -42,6 +42,8 @@
 #  10. leaves NO temp files behind (POST/GET bodies + metadata) on either the
 #      success or the failure path — nested function-scoped RETURN traps do not
 #      clobber each other and every scratch file is removed on all exit paths.
+#  11. accepts the canonical -b/--body flag exactly like the -c/--comment alias
+#      (R1, 2026-08-28): a full verified write via -b alone.
 
 set -euo pipefail
 
@@ -409,11 +411,28 @@ run_comment() {
     seed_state "$mode"
     (
         cd "$REPO_DIR"
+        # Provisioned seats export MOSAIC_GIT_IDENTITY and MOSAIC_BRAIN_HOME
+        # seat-wide (launcher), and both escape this harness's sandboxed HOME:
+        # detect-platform.sh consults MOSAIC_GIT_IDENTITY BEFORE the repo-local
+        # mosaic.gitIdentity pin, and resolves the brain home (whose
+        # fleet/agents presence arms the no-identity fail-loud branch) from
+        # MOSAIC_BRAIN_HOME before $HOME. Without these explicit empties the
+        # wrapper either resolves the REAL seat-slot token (stub curl rejects
+        # it: the documented HTTP 401) or fails loud before any request.
+        # Set-but-empty reads as unset to detect-platform's "${VAR:-}" forms.
+        # NOTE: keep this comment block ABOVE the assignment chain — a comment
+        # inside a backslash-continued prefix chain terminates the command and
+        # silently demotes every earlier assignment to an unexported subshell
+        # assignment (measured 2026-08-28: the wrapper then ran without
+        # MOSAIC_CREDENTIALS_FILE and the suite died at credential resolution
+        # with zero diagnostic output).
         PATH="$BIN_DIR:$PATH" \
         TMPDIR="$TMP_SCRATCH" \
         HOME="$HOME_DIR" \
         XDG_CONFIG_HOME="$XDG_DIR" \
         MOSAIC_CREDENTIALS_FILE="$CREDENTIALS_FILE" \
+        MOSAIC_GIT_IDENTITY="" \
+        MOSAIC_BRAIN_HOME="" \
         ISSUE_COMMENT_TEA_LOG="$TEA_LOG" \
         ISSUE_COMMENT_CURL_LOG="$CURL_LOG" \
         ISSUE_COMMENT_CURL_ARGV_LOG="$CURL_ARGV_LOG" \
@@ -430,7 +449,7 @@ run_comment() {
         ISSUE_COMMENT_REPO_SLUG="$REPO_SLUG" \
         ISSUE_COMMENT_API_BASE="$API_BASE" \
         ISSUE_COMMENT_API_ROOT="$API_ROOT" \
-        "$SCRIPT_DIR/issue-comment.sh" -i "$ISSUE_NUMBER" -c "$BODY" "$@"
+        "$SCRIPT_DIR/issue-comment.sh" -i "$ISSUE_NUMBER" "${BODY_FLAG:--c}" "$BODY" "$@"
     ) > "$OUTPUT_FILE" 2>&1
 }
 
@@ -614,4 +633,21 @@ done
 # issue_url (already exercised by Case 1's fresh-success), so the tightened check
 # is not rejecting genuine writes.
 
+# Case 11 (R1, 2026-08-28): -b/--body is the canonical comment flag and must
+# drive a full verified write exactly like the -c/--comment alias. BODY_FLAG
+# swaps only the flag spelling; every assertion below is case 1's contract.
+BODY_FLAG="-b"
+run_comment fresh-success
+grep -q 'Added and verified comment on Gitea issue #7 (comment ID 51)' "$OUTPUT_FILE"
+grep -q "^POST $API_BASE/issues/7/comments$" "$CURL_LOG"
+if grep -Eq '^comment |^issue comment ' "$TEA_LOG"; then
+    echo "FAIL: --body write went through tea instead of REST" >&2
+    exit 1
+fi
+grep -q "^GET $API_BASE/issues/comments/51$" "$CURL_LOG"
+grep -q "^POST $API_BASE/issues/7/comments $ACTING_LOGIN$" "$AUTH_LOG"
+assert_no_temp_leak "fresh-success-body-flag"
+assert_token_not_in_argv "fresh-success-body-flag"
+unset BODY_FLAG
+
 echo "issue-comment.sh REST create + exact-id read-back regression passed"