fix(hashline-edit): widen non-numeric prefix detection and remove duplicate try-catch

- Replace regex /^([A-Za-z_]+)#.../ with indexOf-based prefix check to catch
  line-ref#VK and line.ref#VK style inputs that were previously giving generic errors
- Extract parseLineRefWithHint helper to eliminate duplicated try-catch in
  validateLineRef and validateLineRefs
- Restore idempotency guard in appendWriteHashlineOutput using new output format
- Add tests for LINE42 extraction, line-ref hint, line.ref hint, and guard behavior

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/hashline-read-enhancer/hook.ts

@@ -136,6 +136,10 @@ function extractFilePath(metadata: unknown): string | undefined {
 }
 
 async function appendWriteHashlineOutput(output: { output: string; metadata: unknown }): Promise<void> {
+  if (output.output.startsWith("File written successfully.")) {
+    return
+  }
+
   const filePath = extractFilePath(output.metadata)
   if (!filePath) {
     return