auto-install claude plugins at harness boot

new hyperhive.claudePlugins NixOS option (list of strings) rendered
to /etc/hyperhive/claude-plugins.json. both hive-ag3nt and hive-m1nd
shell out 'claude plugin install <spec>' for each entry once at
startup before the turn loop opens. failures log a warning but don't
abort boot.

hive-ag3nt/src/lib.rs

@@ -6,6 +6,7 @@ pub mod events;
 pub mod login;
 pub mod login_session;
 pub mod mcp;
+pub mod plugins;
 pub mod turn;
 pub mod web_ui;