fix(list_dir): file-vs-dir mismatch gives an actionable hint (#45)

The 'Not a directory' branch returned a bare message; models re-issued the
identical list_dir until cancel (session a60c54b0). Name the remedy: the path
is a FILE — file_read it or delete/convert it first. Regression test added.

infrastructure/tools/filesystem/src/main/kotlin/com/correx/infrastructure/tools/filesystem/ListDirTool.kt

@@ -111,7 +111,15 @@ class ListDirTool(
                 ToolResult.Failure(request.invocationId, msg, recoverable = true)
             }
             !Files.isDirectory(root) ->
-                ToolResult.Failure(request.invocationId, "Not a directory: $pathString", recoverable = true)
+                // Explicit file-vs-dir mismatch: a bare "Not a directory" left models re-issuing the
+                // identical list_dir until cancel (session a60c54b0). Name the remedy so the next
+                // action is unambiguous — file_read it, or delete/convert it first.
+                ToolResult.Failure(
+                    request.invocationId,
+                    "Not a directory: $pathString is a FILE, not a directory. " +
+                        "Use file_read to read it, or delete/convert it before listing it as a directory.",
+                    recoverable = true,
+                )
             else -> runCatching { walk(root, recursive, request) }.getOrElse {
                 ToolResult.Failure(request.invocationId, "Failed to list dir: ${it.message}", recoverable = false)
             }

infrastructure/tools/filesystem/src/test/kotlin/com/correx/infrastructure/tools/filesystem/ListDirToolTest.kt

@@ -58,6 +58,19 @@ class ListDirToolTest {
         assertFalse(out.contains("deep"), out)
     }
 
+    @Test
+    fun `listing a file gives a recoverable file-vs-dir hint, not a bare error`(): Unit = runBlocking {
+        // Regression (session a60c54b0): list_dir on a plain file returned a bare "Not a directory"
+        // and models re-issued the identical call until cancel. The failure must name the remedy.
+        val root = Files.createTempDirectory("listdir")
+        Files.writeString(root.resolve("pages"), "i am a file")
+        val tool = ListDirTool(allowedPaths = setOf(root), workingDir = root)
+        val result = tool.execute(request(path = "pages")) as ToolResult.Failure
+        assertTrue(result.recoverable, "file-vs-dir mismatch is recoverable")
+        assertTrue(result.reason.contains("FILE"), result.reason)
+        assertTrue(result.reason.contains("file_read"), result.reason)
+    }
+
     @Test
     fun `non-recursive lists only immediate children`(): Unit = runBlocking {
         val root = Files.createTempDirectory("listdir")