[codex] Add marketplace remove command and shared logic (#17752)

## Summary

Move the marketplace remove implementation into shared core logic so
both the CLI command and follow-up app-server RPC can reuse the same
behavior.

This change:
- adds a shared `codex_core::plugins::remove_marketplace(...)` flow
- moves validation, config removal, and installed-root deletion out of
the CLI
- keeps the CLI as a thin wrapper over the shared implementation
- adds focused core coverage for the shared remove path

## Validation

- `just fmt`
- focused local coverage for the shared remove path
- heavier follow-up validation deferred to stacked PR CI

codex-rs/cli/src/main.rs

@@ -1729,6 +1729,15 @@ mod tests {
         assert!(matches!(cli.subcommand, Some(Subcommand::Plugin(_))));
     }
 
+    #[test]
+    fn plugin_marketplace_remove_parses_under_plugin() {
+        let cli =
+            MultitoolCli::try_parse_from(["codex", "plugin", "marketplace", "remove", "debug"])
+                .expect("parse");
+
+        assert!(matches!(cli.subcommand, Some(Subcommand::Plugin(_))));
+    }
+
     #[test]
     fn marketplace_no_longer_parses_at_top_level() {
         let add_result =
@@ -1738,6 +1747,10 @@ mod tests {
         let upgrade_result =
             MultitoolCli::try_parse_from(["codex", "marketplace", "upgrade", "debug"]);
         assert!(upgrade_result.is_err());
+
+        let remove_result =
+            MultitoolCli::try_parse_from(["codex", "marketplace", "remove", "debug"]);
+        assert!(remove_result.is_err());
     }
 
     fn sample_exit_info(conversation_id: Option<&str>, thread_name: Option<&str>) -> AppExitInfo {