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-create.sh

@@ -72,7 +72,7 @@ Examples:
     $(basename "$0") -t "Fix login bug" -l "bug,priority-high"
     $(basename "$0") -t "Add dark mode" -b "Implement theme switching" -m "0.2.0"
 EOF
-    exit 1
+    exit "${1:-1}"
 }
 
 # Parse arguments
@@ -95,7 +95,7 @@ while [[ $# -gt 0 ]]; do
             shift 2
             ;;
         -h|--help)
-            usage
+            usage 0
             ;;
         *)
             echo "Unknown option: $1" >&2