fix(hashline-edit): align autocorrect, BOM/CRLF, and tool description with oh-my-pi

- Rewrite restoreOldWrappedLines to use oh-my-pi's span-scanning algorithm
- Add stripTrailingContinuationTokens and stripMergeOperatorChars helpers
- Fix detectLineEnding to use first-occurrence logic instead of any-match
- Fix applyAppend/applyPrepend to replace empty-line placeholder in empty files
- Enhance tool description with 7 critical rules, tag guidance, and anti-patterns

src/tools/hashline-edit/edit-operation-primitives.ts

@@ -134,6 +134,9 @@ export function applyAppend(lines: string[], text: string | string[]): string[]
   if (normalized.length === 0) {
     throw new Error("append requires non-empty text")
   }
+  if (lines.length === 1 && lines[0] === "") {
+    return [...normalized]
+  }
   return [...lines, ...normalized]
 }
 
@@ -142,6 +145,9 @@ export function applyPrepend(lines: string[], text: string | string[]): string[]
   if (normalized.length === 0) {
     throw new Error("prepend requires non-empty text")
   }
+  if (lines.length === 1 && lines[0] === "") {
+    return [...normalized]
+  }
   return [...normalized, ...lines]
 }