fix: handle single-line hunks and no-newline markers in diff parser

- Hunk headers without comma ("@@ -1 +1 @@") now parse correctly by
  splitting on comma OR space instead of comma only
- Explicit skip for "\ No newline at end of file" lines (was already
  safe but now documents intent)
- Tests added for both edge cases (TDD: tests written first, confirmed
  failure, then fixed)

Addresses sonnet findings #1 and #2 from PR #26 review.

gitea/diff.go

@@ -44,12 +44,16 @@ func ParseDiffNewLines(diff string) *DiffLineRanges {
 			continue
 		}
 
-		// Parse hunk header: @@ -old,count +new,count @@
+		// Parse hunk header: @@ -old,count +new,count @@ or @@ -old +new @@
 		if strings.HasPrefix(line, "@@") && currentFile != "" {
-			// Extract the +N part
+			// Extract the +N part — handle both "+10,8" and "+1" forms
 			parts := strings.Split(line, "+")
 			if len(parts) >= 2 {
-				numStr := strings.Split(parts[1], ",")[0]
+				// Take everything before comma or space
+				numStr := parts[1]
+				if idx := strings.IndexAny(numStr, ", "); idx != -1 {
+					numStr = numStr[:idx]
+				}
 				n, err := strconv.Atoi(numStr)
 				if err == nil {
 					newLine = n
@@ -62,6 +66,11 @@ func ParseDiffNewLines(diff string) *DiffLineRanges {
 			continue
 		}
 
+		// Skip diff metadata lines
+		if strings.HasPrefix(line, "\\") {
+			continue
+		}
+
 		// Count lines in hunk
 		if strings.HasPrefix(line, "+") || strings.HasPrefix(line, " ") {
 			result.files[currentFile][newLine] = true