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/os_compat.c

@@ -1217,3 +1217,40 @@ int unsetenv(const char *name) {
 }
 
 #endif /* _WIN32 */
+
+/* getline(3) itself where the libc has it; the same thing spelled out with
+ * fgetc() where it does not (Windows), so the callers read the same on every
+ * platform. */
+long long os_getline(char **lineptr, size_t *n, FILE *stream) {
+#ifndef _WIN32
+  return (long long)getline(lineptr, n, stream);
+#else
+  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) {
+      const 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 (long long)len;
+#endif
+}