fix(completer): wrap file preview in \sh -c\ for fish compatibility

crates/forge_main/src/completer/input_completer.rs

@@ -40,18 +40,7 @@ impl Completer for InputCompleter {
                 .map(|file| file.path)
                 .collect();
 
-            // Preview command: show directory listing for dirs, file contents for files.
-            // {2} references the path column (items are formatted as "{idx}\t{path}").
-            // Use bat for syntax-highlighted file previews when available, falling back
-            // to cat. Mirrors the shell plugin's _FORGE_CAT_CMD and completion.zsh preview.
-            let cat_cmd = if which_bat() {
-                "bat --color=always --style=numbers,changes --line-range=:500"
-            } else {
-                "cat"
-            };
-            let preview_cmd = format!(
-                "if [ -d {{2}} ]; then ls -la --color=always {{2}} 2>/dev/null || ls -la {{2}}; else {cat_cmd} {{2}}; fi"
-            );
+            let preview_cmd = build_preview_cmd(cat_cmd_for_preview());
 
             let mut builder = ForgeWidget::select("File", files)
                 .with_preview(preview_cmd)
@@ -87,3 +76,47 @@ fn which_bat() -> bool {
         .map(|o| o.status.success())
         .unwrap_or(false)
 }
+
+/// Prefers `bat` for syntax-highlighted output when it is on `$PATH`, falling
+/// back to plain `cat`. Mirrors the shell plugin's `_FORGE_CAT_CMD`.
+fn cat_cmd_for_preview() -> &'static str {
+    if which_bat() {
+        "bat --color=always --style=numbers,changes --line-range=:500"
+    } else {
+        "cat"
+    }
+}
+
+/// Builds the fzf `--preview` command as `sh -c '…' _ {2}`.
+///
+/// Wrapping in `sh -c` is required because fzf dispatches previews through
+/// `$SHELL`, and shells like fish cannot parse the POSIX `if/then/fi` body.
+/// `{2}` is fzf's substitution for the tab-separated path column.
+fn build_preview_cmd(cat_cmd: &str) -> String {
+    format!(
+        r#"sh -c 'if [ -d "$1" ]; then ls -la --color=always "$1" 2>/dev/null || ls -la "$1"; else {cat_cmd} "$1"; fi' _ {{2}}"#
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+
+    use super::*;
+
+    #[test]
+    fn test_build_preview_cmd_with_cat() {
+        let fixture = "cat";
+        let actual = build_preview_cmd(fixture);
+        let expected = r#"sh -c 'if [ -d "$1" ]; then ls -la --color=always "$1" 2>/dev/null || ls -la "$1"; else cat "$1"; fi' _ {2}"#;
+        assert_eq!(actual, expected);
+    }
+
+    #[test]
+    fn test_build_preview_cmd_with_bat() {
+        let fixture = "bat --color=always --style=numbers,changes --line-range=:500";
+        let actual = build_preview_cmd(fixture);
+        let expected = r#"sh -c 'if [ -d "$1" ]; then ls -la --color=always "$1" 2>/dev/null || ls -la "$1"; else bat --color=always --style=numbers,changes --line-range=:500 "$1"; fi' _ {2}"#;
+        assert_eq!(actual, expected);
+    }
+}