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.

include/os_compat.h

@@ -2,6 +2,7 @@
 #define CST_OS_COMPAT_H
 
 #include <stddef.h>
+#include <stdio.h>
 #include <time.h>
 
 #ifdef __cplusplus
@@ -12,6 +13,13 @@ extern "C" {
 #define OS_PATH_MAX 4096
 #endif
 
+/* --- text --- */
+/* One line of a file into *lineptr, growing it as needed: POSIX getline(3),
+ * which a MinGW CRT does not have (the session file readers used it directly
+ * and only the Windows build noticed). Returns the number of bytes read -- 0
+ * for an empty line -- or -1 at the end of the file. */
+long long os_getline(char **lineptr, size_t *n, FILE *stream);
+
 /* --- path --- */
 int os_path_is_abs(const char *p);
 /* Join with native separator; out always uses '/' on Unix and '\\' on Win for