fix(hashline-edit): address Oracle review feedback

- Extract WRITE_SUCCESS_MARKER constant to couple guard and output string
- Remove double blank line after parseLineRefWithHint
- Add comment clarifying normalized equals ref.trim() in error paths

src/tools/hashline-edit/validation.ts

@@ -38,6 +38,7 @@ export function parseLineRef(ref: string): LineRef {
       hash: match[2],
     }
   }
+  // normalized equals ref.trim() in all error paths — extraction only succeeds for valid refs
   const hashIdx = normalized.indexOf('#')
   if (hashIdx > 0) {
     const prefix = normalized.slice(0, hashIdx)
@@ -151,7 +152,6 @@ function parseLineRefWithHint(ref: string, lines: string[]): LineRef {
   }
 }
 
-
 export function validateLineRefs(lines: string[], refs: string[]): void {
   const mismatches: HashMismatch[] = []