[codex] Preserve remote plugin directory order

[jameswt-oai]

Summary

• preserve the plugin directory endpoint's response order while merging installed state
• append unmatched installed-only plugins afterward when requested
• add focused coverage for directory order and installed-only placement

Why

The remote marketplace merge currently reconstructs plugins through ordered maps and sets, then sorts the result alphabetically by display name. That discards any ordering supplied by the plugin directory endpoint before the list reaches Desktop.

Implementation

Directory plugin IDs are unique, so the merge now iterates the directory vector directly in response order. For each directory plugin, it removes matching installed state from an ID-indexed map and builds the summary. Any entries left in the installed map are installed-only plugins and are appended when include_installed_only is enabled.

There is no separate rank field, rank map, or final sort. Desktop therefore receives directory order—including any backend ranking—and can preserve it within its existing stable UI state tiers.

Testing

• just test -p codex-core-plugins (225 passed)

[xl-openai]

One alternative to consider: instead of recording an order map and sorting at the end, we could build the result directly in directory_plugins order, then append installed-only plugins. That might make the code a bit simpler and more explicit, but the current approach is a reasonable small fix.