exercises: add Exercise 08 SystemStartup scaffold with early SPI deselect and SD/OLED startup orchestration

exercises/08_SystemStartup/README.md

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+## Exercise 08: SystemStartup Package Scaffold
+
+This exercise starts a reusable `SystemStartup` package that is intended to be shared by future exercises and field firmware.
+
+Current package responsibilities:
+
+1. Initialize OLED and show boot/status messages.
+2. Initialize SD startup watcher.
+3. Keep SD monitoring active in `loop()` with a single call.
+
+Current integration pattern:
+
+```cpp
+#include "SystemStartup.h"
+
+static SystemStartup g_systemStartup(Serial);
+
+void setup() {
+  Serial.begin(115200);
+  g_systemStartup.begin();
+}
+
+void loop() {
+  g_systemStartup.update();
+  delay(10);
+}
+```
+
+This is the foundation for adding more startup subsystems (RTC sync/check, etc.) behind the same `begin()/update()` API.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```