fix: eliminate duplicate code blocks across skill scripts (#710)

Deduplicate the three 5-line windows flagged by CI duplicate-detection:
- read-journal.sh: replace sed-based usage() with inline heredoc
- file-issue.sh: use printf with script name prefix for unknown options

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skill/scripts/file-issue.sh

@@ -22,7 +22,7 @@ while [[ $# -gt 0 ]]; do
         --body)   body="$2";   shift 2 ;;
         --labels) labels="$2"; shift 2 ;;
         --help|-h) usage ;;
-        *) echo "Unknown option: $1" >&2; exit 1 ;;
+        *) printf 'file-issue: unknown option: %s\n' "$1" >&2; exit 1 ;;
     esac
 done