docs: extract focus encoding example into standalone project

Extract the inline code example from focus.h into a standalone
buildable example at example/c-vt-encode-focus. The header now
uses a Doxygen @snippet tag to include the code from the example
source file, so the documentation stays in sync with code that
is verified to compile and run.

example/c-vt-encode-focus/src/main.c

@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <ghostty/vt.h>
+
+//! [focus-encode]
+int main() {
+  char buf[8];
+  size_t written = 0;
+
+  GhosttyResult result = ghostty_focus_encode(
+      GHOSTTY_FOCUS_GAINED, buf, sizeof(buf), &written);
+
+  if (result == GHOSTTY_SUCCESS) {
+    printf("Encoded %zu bytes: ", written);
+    fwrite(buf, 1, written, stdout);
+    printf("\n");
+  }
+
+  return 0;
+}
+//! [focus-encode]