framework tools/git: lookup-pipeline pipefail + provider-normalization test arm (codex round on PR #1464)

- issue-assign + milestone-create: set -o pipefail; the remove-assignee
  lookup pipeline now fails loud on provider failure instead of reading
  an empty result as a silent no-assignees skip (codex blocker). A
  successful lookup with zero assignees still skips the edit.
- test-issue-assign-usage-contract: 6b arm proves provider-exit
  normalization end to end (gh stub exiting 2 on issue edit -> wrapper
  exit 1 with the normalized stderr message; codex should-fix).
- Battery: 23/23 suites green.

packages/mosaic/framework/tools/git/issue-assign.sh

@@ -3,6 +3,7 @@
 # Usage: issue-assign.sh -i ISSUE_NUMBER [-a assignee] [-l labels] [-m milestone]
 
 set -e
+set -o pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/detect-platform.sh"
@@ -96,7 +97,14 @@ case "$PLATFORM" in
         fi
         if [[ "$REMOVE_ASSIGNEE" == true ]]; then
             # Get current assignees and remove them
-            CURRENT=$(gh issue view "$ISSUE" --json assignees -q '.assignees[].login' 2>/dev/null | tr '\n' ',')
+            # pipefail preserves the provider status through the pipeline;
+            # a FAILED lookup exits here instead of reading as a silent
+            # no-assignees skip (codex PR #1464). A successful lookup with
+            # zero assignees still skips the edit below.
+            CURRENT=$(gh issue view "$ISSUE" --json assignees -q '.assignees[].login' 2>/dev/null | tr '\n' ',') || {
+                echo "Error: could not read current assignees (provider lookup failed)" >&2
+                exit 1
+            }
             if [[ -n "$CURRENT" ]]; then
                 prov_rc=0
                 gh issue edit "$ISSUE" --remove-assignee "${CURRENT%,}" || prov_rc=$?