fix(cortex-cli): show alternative workflow when removing built-in agent

Fixes bounty issue #1666

When a user tries to remove a built-in agent, the error message now suggests
creating a custom copy using 'cortex agent create' or manually copying the
agent to ~/.cortex/agents/ where it can be modified.

Author: factorydroid

cortex-cli/src/agent_cmd.rs

@@ -1135,7 +1135,13 @@ async fn run_remove(args: RemoveArgs) -> Result<()> {
         .ok_or_else(|| anyhow::anyhow!("Agent '{}' not found", args.name))?;
 
     if agent.native {
-        bail!("Cannot remove built-in agent '{}'", args.name);
+        bail!(
+            "Cannot remove built-in agent '{}'. \
+            Tip: You can create a custom version of this agent using \
+            'cortex agent create' with similar settings, or copy it to \
+            ~/.cortex/agents/ and modify it there.",
+            args.name
+        );
     }
 
     let path = agent