Sync remote installed plugin bundles (#20268)

## Summary
- Download missing remote installed plugin bundles during app-server
startup and plugin/list refresh.
- Upgrade cached remote installed bundles when the backend installed
version changes.
- Remove stale remote installed bundle caches without writing remote
plugin state into config.toml.

## Review note
This is a clean PR branch cut from the current diff on top of latest
`origin/main`. The diff intentionally has no `codex-rs/core/**` files,
so CODEOWNERS should not request the core-directory owner review from
stale PR history.

## Validation
Already run on the source branch before creating this clean PR:
- `just fmt`
- `cargo test -p codex-core-plugins`
- `cargo test -p codex-app-server --test all
app_server_startup_sync_downloads_remote_installed_plugin_bundles --
--nocapture`
- `cargo test -p codex-app-server --test all
plugin_list_sync_upgrades_and_removes_remote_installed_plugin_bundles --
--nocapture`
- `cargo test -p codex-app-server --test all
app_server_startup_remote_plugin_sync_runs_once -- --nocapture`
- `just fix -p codex-core-plugins`
- `just fix -p codex-app-server`
- `git diff --check`

codex-rs/app-server/src/codex_message_processor/plugins.rs

@@ -541,6 +541,14 @@ impl CodexMessageProcessor {
             )));
         }
         let actual_remote_marketplace_name = remote_detail.marketplace_name.clone();
+        // Direct install writes the same cache tree that installed-plugin sync
+        // prunes before the backend installed snapshot can include this plugin.
+        let _remote_plugin_cache_mutation =
+            codex_core_plugins::remote::mark_remote_plugin_cache_mutation_in_flight(
+                config.codex_home.as_path(),
+                &actual_remote_marketplace_name,
+                &remote_detail.summary.name,
+            );
         let validated_bundle = codex_core_plugins::remote_bundle::validate_remote_plugin_bundle(
             &plugin_name,
             &actual_remote_marketplace_name,