fix: resolve --agent override via config alias before raw command

When --agent is passed to uwf thread exec, try config.agents[alias]
first (e.g. 'hermes' → config.agents.hermes = {command: 'uwf-hermes'}),
then fall back to parseAgentOverride for raw command names.

Also change eval CLI default --agent from 'hermes' to 'uwf-hermes'
so it works without config alias lookup.

Refs #91

packages/cli/src/commands/thread.ts

@@ -961,6 +961,12 @@ function resolveAgentConfig(
   agentOverride: string | null,
 ): AgentConfig {
   if (agentOverride !== null) {
+    // Try config alias first (e.g. "hermes" → config.agents.hermes),
+    // then fall back to raw command name (e.g. "uwf-hermes" or "/usr/bin/agent").
+    const fromAlias = config.agents[agentOverride as AgentAlias];
+    if (fromAlias !== undefined) {
+      return fromAlias;
+    }
     return parseAgentOverride(agentOverride);
   }