refactor(cli): simplify isBinaryFile check to UTF-16 BOM only

Per review: CJK and legacy single-byte encodings already passed the old
control-char heuristic (their bytes are all >= 0x80, never NUL), so the
full Encoding.detect pass in isBinaryFile was solving a non-problem for
them. The only realistic regression in the old heuristic is UTF-16 with
BOM, where the second byte of every ASCII character is 0x00 and fires
the NUL-byte early-return.

Replace the detect call with a 2-byte BOM check via a new
Encoding.hasUtf16Bom helper. Also reword the move/add inline markers in
patch/index.ts to note that Encoding.write handles mkdir.

packages/opencode/src/patch/index.ts

@@ -549,7 +549,7 @@ export namespace Patch {
 
           if (hunk.move_path) {
             // Handle file move
-            await Encoding.write(hunk.move_path, fileUpdate.content, fileUpdate.encoding) // kilocode_change
+            await Encoding.write(hunk.move_path, fileUpdate.content, fileUpdate.encoding) // kilocode_change - encoding-aware write (mkdirs)
             await fs.unlink(hunk.path)
             modified.push(hunk.move_path)
             log.info(`Moved file: ${hunk.path} -> ${hunk.move_path}`)