fix: embed firmware version from version.txt, log at boot (#354)

- Add version.txt (0.6.0) read by CMakeLists.txt so
  esp_app_get_description()->version matches the release tag
- Log firmware version on boot: "v0.6.0 — Node ID: X"
- Remove stale Kconfig help text (said default 2.0, actual is 15.0)

Fixes the version mismatch reported in #354 where flashing v0.5.3
binaries showed v0.4.3 because PROJECT_VER was never set.

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/main/main.c

@@ -16,6 +16,7 @@
 #include "esp_event.h"
 #include "esp_log.h"
 #include "nvs_flash.h"
+#include "esp_app_desc.h"
 #include "sdkconfig.h"
 
 #include "csi_collector.h"
@@ -137,7 +138,9 @@ void app_main(void)
     /* Load runtime config (NVS overrides Kconfig defaults) */
     nvs_config_load(&g_nvs_config);
 
-    ESP_LOGI(TAG, "ESP32-S3 CSI Node (ADR-018) — Node ID: %d", g_nvs_config.node_id);
+    const esp_app_desc_t *app_desc = esp_app_get_description();
+    ESP_LOGI(TAG, "ESP32-S3 CSI Node (ADR-018) — v%s — Node ID: %d",
+             app_desc->version, g_nvs_config.node_id);
 
     /* Initialize WiFi STA (skip entirely under QEMU mock — no RF hardware) */
 #ifndef CONFIG_CSI_MOCK_SKIP_WIFI_CONNECT