Add libc implementation stubs

Real string core and process teardown; thin read/write/close/getpid/_exit syscall wrappers; the rest follows the stub convention (documented error return, errno = ENOSYS, TODO). syscall.c is the only kernel ABI surface; crt0.S is the static entry point, kept out of libc.a.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

src/unistd.c

@@ -0,0 +1,47 @@
+/*
+ * unistd.c — thin wrappers around raw syscalls.
+ *
+ * Each function here is a one-liner on purpose: the kernel is the API
+ * (project guideline #6), and a wrapper that does more than translate
+ * arguments is a wrapper that can lie. Error translation (kernel -errno
+ * -> errno) happens inside syscall() itself.
+ */
+
+#include <errno.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+ssize_t read(int fd, void *buf, size_t count)
+{
+    return (ssize_t)syscall(SYS_read, fd, buf, count);
+}
+
+ssize_t write(int fd, const void *buf, size_t count)
+{
+    return (ssize_t)syscall(SYS_write, fd, buf, count);
+}
+
+int close(int fd)
+{
+    return (int)syscall(SYS_close, fd);
+}
+
+pid_t getpid(void)
+{
+    return (pid_t)syscall(SYS_getpid);
+}
+
+void _exit(int status)
+{
+    syscall(SYS_exit, status);
+    for (;;)
+        ; /* unreachable */
+}
+
+/* Stub: trivially a one-liner once wanted (SYS_unlink = 87 on x86_64). */
+int unlink(const char *path)
+{
+    (void)path;
+    errno = ENOSYS;
+    return -1;
+}