fix(msgfmt): address review — extract border helper, add slim box test, drop plans

- Extract containsHorizontalBorder() to collapse 6 inline border
  checks into one — no more copy-paste miscounts on 15-char Unicode
  strings
- Add msg_slim_dashed_box.txt fixture with │ prompt to exercise
  findGenericSlimMessageBox's ╌ path (previously untested because
  the > fixture hit findGreaterThanMessageBox first)
- Rename screen → message in test lambda for consistency
- Remove docs/plans/ from the branch

Author: johnstcn

docs/plans/issue-209-research.md

@@ -4,6 +4,13 @@ import (
 	"strings"
 )
 
+// containsHorizontalBorder reports whether the line contains a
+// horizontal border made of box-drawing characters (─ or ╌).
+func containsHorizontalBorder(line string) bool {
+	return strings.Contains(line, "───────────────") ||
+		strings.Contains(line, "╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌")
+}
+
 // Usually something like
 // ───────────────  (or ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌)
 // >
@@ -12,7 +19,7 @@ import (
 func findGreaterThanMessageBox(lines []string) int {
 	for i := len(lines) - 1; i >= max(len(lines)-6, 0); i-- {
 		if strings.Contains(lines[i], ">") {
-			if i > 0 && (strings.Contains(lines[i-1], "───────────────") || strings.Contains(lines[i-1], "╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌")) {
+			if i > 0 && containsHorizontalBorder(lines[i-1]) {
 				return i - 1
 			}
 			return i
@@ -27,9 +34,9 @@ func findGreaterThanMessageBox(lines []string) int {
 // ───────────────  (or ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌)
 func findGenericSlimMessageBox(lines []string) int {
 	for i := len(lines) - 3; i >= max(len(lines)-9, 0); i-- {
-		if (strings.Contains(lines[i], "───────────────") || strings.Contains(lines[i], "╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌")) &&
+		if containsHorizontalBorder(lines[i]) &&
 			(strings.Contains(lines[i+1], "|") || strings.Contains(lines[i+1], "│") || strings.Contains(lines[i+1], "❯")) &&
-			(strings.Contains(lines[i+2], "───────────────") || strings.Contains(lines[i+2], "╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌")) {
+			containsHorizontalBorder(lines[i+2]) {
 			return i
 		}
 	}