fix(tools/git): -h/--help now exits 0 across 7 wrappers (#701)

usage() hard-coded exit 1, so every --help/-h invocation on
issue-assign, issue-create, issue-list, milestone-create, pr-create,
pr-list, and pr-merge exited nonzero, logging phantom isError records
across fleet lanes and breaking set -e composition. usage() now takes
an exit-code arg (default 1); the -h|--help case calls usage 0. All
other error paths are untouched.

Closes #701

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

@@ -33,7 +33,7 @@ Examples:
     $(basename "$0") -i 42 -l "in-progress" -m "0.2.0"
     $(basename "$0") -i 42 -a @me
 EOF
-    exit 1
+    exit "${1:-1}"
 }
 
 # Parse arguments
@@ -60,7 +60,7 @@ while [[ $# -gt 0 ]]; do
             shift
             ;;
         -h|--help)
-            usage
+            usage 0
             ;;
         *)
             echo "Unknown option: $1" >&2