Implement all @copilot Makefile and buffer test annotations

- Defined V0_SRC_DEPS and V0_TEST_DEPS in v0/include.mk
- Updated clean rule to remove dependency files
- Referenced dependency variables for .d includes
- Added and deduplicated file-read test in v0/test_buffer.c

All tests pass. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

v0/test_buffer.c

@@ -2,9 +2,6 @@
 #include "buffer.h"
 #include <stdio.h>
 
-// @copilot add test for reading from a file.
-// Create test_buffer_xyz.txt if you need multiple,
-// otherwise simply test_buffer.txt
 
 static void test_buffer_string_reads_chars(void) {
     Buffer* buf = buffer_open_string("hi");