fix(coding-agent): corrected off-by-one range errors in hashline edits

- Added auto-correction logic for off-by-one range start errors in hashline edits.
- Added safety check to prevent false-positive auto-correction when position already includes boundary.
- Added test coverage for off-by-one range correction and duplicate leading line handling.
- Extracted turn-aborted-guidance prompt to external markdown file for better maintainability.

packages/coding-agent/src/patch/hashline.ts

@@ -458,6 +458,18 @@ function maybeWarnSuspiciousUnicodeEscapePlaceholder(edits: HashlineEdit[], warn
 // Edit Application
 // ═══════════════════════════════════════════════════════════════════════════
 
+const MIN_AUTOCORRECT_LENGTH = 2;
+
+function shouldAutocorrect(line: string, otherLine: string): boolean {
+	if (!line || line !== otherLine) return false;
+	line = line.trim();
+	if (line.length < MIN_AUTOCORRECT_LENGTH) {
+		// if brace, we allow
+		return line.endsWith("}") || line.endsWith(")");
+	}
+	return true;
+}
+
 /**
  * Apply an array of hashline edits to file content.
  *
@@ -630,9 +642,7 @@ export function applyHashlineEdits(
 					const trailingReplacementLine = newLines[newLines.length - 1]?.trimEnd();
 					const nextSurvivingLine = fileLines[edit.end.line]?.trimEnd();
 					if (
-						trailingReplacementLine &&
-						nextSurvivingLine &&
-						trailingReplacementLine === nextSurvivingLine &&
+						shouldAutocorrect(trailingReplacementLine, nextSurvivingLine) &&
 						// Safety: only correct when end-line content differs from the duplicate.
 						// If end already points to the boundary, matching next line is coincidence.
 						fileLines[edit.end.line - 1]?.trimEnd() !== trailingReplacementLine
@@ -642,6 +652,19 @@ export function applyHashlineEdits(
 							`Auto-corrected range replace ${edit.pos.line}#${edit.pos.hash}-${edit.end.line}#${edit.end.hash}: removed trailing replacement line "${trailingReplacementLine}" that duplicated next surviving line`,
 						);
 					}
+					const leadingReplacementLine = newLines[0]?.trimEnd();
+					const prevSurvivingLine = fileLines[edit.pos.line - 2]?.trimEnd();
+					if (
+						shouldAutocorrect(leadingReplacementLine, prevSurvivingLine) &&
+						// Safety: only correct when pos-line content differs from the duplicate.
+						// If pos already points to the boundary, matching prev line is coincidence.
+						fileLines[edit.pos.line - 1]?.trimEnd() !== leadingReplacementLine
+					) {
+						newLines.shift();
+						warnings.push(
+							`Auto-corrected range replace ${edit.pos.line}#${edit.pos.hash}-${edit.end.line}#${edit.end.hash}: removed leading replacement line "${leadingReplacementLine}" that duplicated preceding surviving line`,
+						);
+					}
 					fileLines.splice(edit.pos.line - 1, count, ...newLines);
 					trackFirstChanged(edit.pos.line);
 				}