pass SYSTEM.md as --append-system-prompt instead of relying on shard to Read it

src/shard.rs

@@ -98,6 +98,14 @@ impl ShardSession {
     pub async fn spawn(cfg: SpawnConfig<'_>) -> anyhow::Result<Self> {
         let identity_dir = paths::identity_dir();
 
+        // SYSTEM.md is the harness contract. Claude Code only auto-loads
+        // CLAUDE.md from --add-dir, so SYSTEM.md gets passed explicitly via
+        // --append-system-prompt to ensure the shard always sees it.
+        let system_md_path = identity_dir.join("SYSTEM.md");
+        let system_md = tokio::fs::read_to_string(&system_md_path)
+            .await
+            .with_context(|| format!("reading {}", system_md_path.display()))?;
+
         let mut cmd = Command::new("claude");
         cmd.args([
             "--print",
@@ -114,6 +122,8 @@ impl ShardSession {
             cfg.allowed_tools,
             "--mcp-config",
             &cfg.mcp_config_path.to_string_lossy(),
+            "--append-system-prompt",
+            &system_md,
         ]);
         cmd.current_dir(&identity_dir);
         cmd.stdin(Stdio::piped());