feat: add command queue and FIFO pipe for unified input handling

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/pipe.h

@@ -0,0 +1,9 @@
+#ifndef PIPE_H
+#define PIPE_H
+
+/* Start the FIFO reader thread.
+ * Creates /tmp/looper_cmd (or aborts on error).
+ * Returns 0 on success, -1 on failure. */
+int pipe_start_reader(void);
+
+#endif