fix(cli): drop the POSIX probe-counter store that Clang 23 rejects

`g_mcp_command_path_probe_counter` is only ever read inside
`cbm_json_mcp_probe_command_path`, which is compiled for Windows alone.
On POSIX test builds the setter stored a pointer nothing read, and Clang
23 (Homebrew LLVM 23.1.0, which `brew install llvm` now pours on the
macOS LSan leg) flags that as `-Wunused-but-set-global`; with `-Werror`
the whole test-runner build dies at `src/cli/cli.c:1768:21`.

Keep the counter where a probe can happen (Windows) and make the POSIX
setter an explicit no-op with the reason spelled out. The three tests
that install a counter keep asserting it stays at zero on every platform,
which on POSIX is exactly what "no classifier" means.

Verified in the failing arena: `origin/main` `src/cli/cli.c` fails under
clang 23.1.1 with the CI diagnostic; with this change it compiles clean,
as do all 105 built `src` TUs and all 155 test + extraction TUs under the
same flags. `cli` suite: 292 passed with Homebrew LLVM 22.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

src/cli/cli.c

@@ -1765,14 +1765,25 @@ static bool cbm_json_mcp_command_path_probe_safe(const char *command) {
 #endif
 
 #ifdef CBM_CLI_ENABLE_TEST_API
+#ifdef _WIN32
 static CBM_TLS int *g_mcp_command_path_probe_counter = NULL;
+#endif
 
 bool cbm_mcp_command_path_probe_safe_for_testing(const char *command, bool windows) {
     return cbm_json_mcp_command_path_probe_safe_for_platform(command, windows);
 }
 
 void cbm_set_mcp_command_path_probe_counter_for_testing(int *counter) {
+#ifdef _WIN32
     g_mcp_command_path_probe_counter = counter;
+#else
+    /* The command-path classifier (cbm_json_mcp_probe_command_path) is compiled
+     * for Windows only, so on POSIX there is no probe to count. The tests still
+     * install a counter on every platform and assert it stays at zero; a pointer
+     * stored here but never read is what Clang 23 rejects under -Werror
+     * (-Wunused-but-set-global). */
+    (void)counter;
+#endif
 }
 #endif