the session readers use getline(3), which a Windows CRT does not have

Building the Windows TUI by hand -- the first local build since the CI file
went away -- stopped in src/tools/session_tools.c: `getline` was an implicit
declaration there, which this GCC treats as an error. src/session.c had a
private fgetc() shim for exactly that; the session-list scan is newer and used
the POSIX call directly, and only the Windows build notices.

The shim moves to os_compat as os_getline(): getline(3) itself on Unix, the
fgetc() loop on Windows, one prototype in the header so a caller reads the same
everywhere. src/session.c drops its copy and calls it; the scan does too (its
`ssize_t` becomes `long long`, which Windows has).

make check is green on Linux, the Windows cross build now finishes and
packages (dist/cst-agent-0.9.7-windows-x86_64.zip plus the NSIS setup), and the
static Linux tarball was rebuilt with it.

src/session.c

@@ -15,39 +15,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-#ifdef _WIN32
-typedef long long cst_ssize_t;
-static cst_ssize_t cst_getline(char **lineptr, size_t *n, FILE *stream) {
-  if (!lineptr || !n || !stream)
-    return -1;
-  if (!*lineptr || *n == 0) {
-    *n = 256;
-    *lineptr = (char *)malloc(*n);
-    if (!*lineptr)
-      return -1;
-  }
-  size_t len = 0;
-  int c = 0;
-  while ((c = fgetc(stream)) != EOF) {
-    if (len + 2 > *n) {
-      size_t nn = (*n) * 2;
-      char *nb = (char *)realloc(*lineptr, nn);
-      if (!nb)
-        return -1;
-      *lineptr = nb;
-      *n = nn;
-    }
-    (*lineptr)[len++] = (char)c;
-    if (c == '\n')
-      break;
-  }
-  if (len == 0 && c == EOF)
-    return -1;
-  (*lineptr)[len] = 0;
-  return (cst_ssize_t)len;
-}
-#define getline cst_getline
-#endif
+#include "os_compat.h"
 
 struct cst_session {
   char path[4096];
@@ -158,7 +126,7 @@ static void load_existing(cst_session_t *s) {
     return;
   char *line = NULL;
   size_t cap = 0;
-  while (getline(&line, &cap, rp) != -1) {
+  while (os_getline(&line, &cap, rp) != -1) {
     cJSON *o = cJSON_Parse(line);
     if (!o || !cJSON_IsObject(o)) {
       cJSON_Delete(o);