harness: declarative claude plugin marketplaces

new `hyperhive.claudeMarketplaces` option (list of strings — URL,
path, or github:owner/repo). harness boot adds each via
`claude plugin marketplace add` before updating + installing the
configured plugins, so specs like `foo@some-marketplace` resolve
on a fresh container. idempotent: 'already exists' stderr is
treated as success.

hive-ag3nt/src/plugins.rs

@@ -18,6 +18,53 @@ use tokio::process::Command;
 use crate::client;
 
 const PLUGINS_PATH: &str = "/etc/hyperhive/claude-plugins.json";
+const MARKETPLACES_PATH: &str = "/etc/hyperhive/claude-marketplaces.json";
+
+/// Add every marketplace from `/etc/hyperhive/claude-marketplaces.json`
+/// via `claude plugin marketplace add <source>`. Idempotent: re-add of
+/// an existing marketplace is treated as success (claude prints an
+/// "already exists" message and exits non-zero on some versions).
+/// Required before any `<plugin>@<marketplace>` install can resolve.
+async fn add_marketplaces() {
+    let raw = match tokio::fs::read_to_string(MARKETPLACES_PATH).await {
+        Ok(s) => s,
+        Err(_) => return,
+    };
+    let sources: Vec<String> = match serde_json::from_str(&raw) {
+        Ok(v) => v,
+        Err(e) => {
+            tracing::warn!(path = MARKETPLACES_PATH, error = ?e, "claude-marketplaces spec parse failed; skipping");
+            return;
+        }
+    };
+    for source in sources {
+        match Command::new("claude")
+            .args(["plugin", "marketplace", "add", &source])
+            .output()
+            .await
+        {
+            Ok(out) if out.status.success() => {
+                tracing::info!(source = %source, "claude plugin marketplace add ok");
+            }
+            Ok(out) => {
+                let stderr = String::from_utf8_lossy(&out.stderr);
+                if stderr.contains("already") {
+                    tracing::debug!(source = %source, "marketplace already added");
+                } else {
+                    tracing::warn!(
+                        source = %source,
+                        status = ?out.status,
+                        stderr = %stderr,
+                        "claude plugin marketplace add failed (non-fatal)",
+                    );
+                }
+            }
+            Err(e) => {
+                tracing::warn!(source = %source, error = ?e, "claude plugin marketplace add spawn failed");
+            }
+        }
+    }
+}
 
 /// Update all configured plugin marketplaces. Non-fatal — logs a warning
 /// on failure but does not abort the install sequence.
@@ -64,6 +111,7 @@ pub async fn install_configured(socket: &Path, notify_recipient: Option<&str>) {
     if specs.is_empty() {
         return;
     }
+    add_marketplaces().await;
     update_marketplaces().await;
     for spec in specs {
         match Command::new("claude")