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/milestone-create.sh

@@ -3,6 +3,7 @@
 # Usage: milestone-create.sh -t "Title" [-d "Description"] [--due "YYYY-MM-DD"]
 
 set -e
+set -o pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/detect-platform.sh"