feat: add system monitoring RPC endpoints (system.stats, system.processes, system.temperature)

Read real metrics from /proc/stat, /proc/meminfo, /proc/uptime, /proc/loadavg,
df, ps, and /sys/class/thermal/. CPU usage computed via dual-sample jiffies.
Deployed and verified on live server — all three endpoints return real data.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/api/rpc/mod.rs

@@ -15,6 +15,7 @@ mod peers;
 mod router;
 mod tor;
 mod totp;
+mod system;
 mod update;
 mod wallet;
 
@@ -319,6 +320,11 @@ impl RpcHandler {
             "dwn.status" => self.handle_dwn_status().await,
             "dwn.sync" => self.handle_dwn_sync().await,
 
+            // System monitoring
+            "system.stats" => self.handle_system_stats().await,
+            "system.processes" => self.handle_system_processes().await,
+            "system.temperature" => self.handle_system_temperature().await,
+
             // System updates
             "update.check" => self.handle_update_check().await,
             "update.status" => self.handle_update_status().await,