Move marketplace add under plugin command (#18116)

## Summary
- move the marketplace add CLI from `codex marketplace add` to `codex
plugin marketplace add`
- keep marketplace config overrides working through the nested plugin
command
- reject `--sparse` for local marketplace directory sources before the
local-source install path bypasses git-source validation

## Validation
- `just fmt`
- `git diff --check`
- `cargo test -p codex-cli`
- `cargo test -p codex-core marketplace_add -- --nocapture`
- `cargo test -p codex-core
install_plugin_updates_config_with_relative_path_and_plugin_key --
--nocapture`
- `xli-test-marketplace-cli` local isolated matrix: `T1`, `L1`-`L10`

codex-rs/cli/tests/marketplace_upgrade.rs

@@ -0,0 +1,36 @@
+use anyhow::Result;
+use predicates::str::contains;
+use std::path::Path;
+use tempfile::TempDir;
+
+fn codex_command(codex_home: &Path) -> Result<assert_cmd::Command> {
+    let mut cmd = assert_cmd::Command::new(codex_utils_cargo_bin::cargo_bin("codex")?);
+    cmd.env("CODEX_HOME", codex_home);
+    Ok(cmd)
+}
+
+#[tokio::test]
+async fn marketplace_upgrade_runs_under_plugin() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    codex_command(codex_home.path())?
+        .args(["plugin", "marketplace", "upgrade"])
+        .assert()
+        .success()
+        .stdout(contains("No configured Git marketplaces to upgrade."));
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn marketplace_upgrade_no_longer_runs_at_top_level() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    codex_command(codex_home.path())?
+        .args(["marketplace", "upgrade"])
+        .assert()
+        .failure()
+        .stderr(contains("unexpected argument 'upgrade' found"));
+
+    Ok(())
+}