Unified mentions in TUI (#19068)

This PR replaces the TUI’s file-only `@mention` popup with a unified
mentions experience. Typing `@...` now searches across filesystem
matches, installed plugins, and skills in one popup, with result types
clearly labeled and selectable from the same flow.

- Adds a unified `@mentions` popup that returns:
  - plugins
  - skills
  - files
  - directories

- Adds search modes so users can narrow the popup without changing their
query:
  - All Results _(default/same as Codex App)_
  - Filesystem Only
  - Plugins _(...and skills)_

- Preserves existing insertion behavior:
  - selected file paths are inserted into the prompt
  - paths with spaces are quoted
  - image file selections still attach as images when possible
  - selecting a plugin or skill inserts the corresponding `$name`
- the composer records the canonical mention binding, such as
`plugin://...` or the skill path

- Expanded `@mentions` rendering:
  - type tags for Plugin, Skill, File, and Dir
  - distinct plugin/filesystem colors
  - stable fixed-height layout (8 rows)
  - truncation behavior for narrow terminals

Note:
- The unified mentions popup does not display app connectors under
`@mention` results for Codex App parity. Connector mentions remain
available through the existing `$mention` path.


https://github.com/user-attachments/assets/f93781ed-57d3-4cb5-9972-675bc5f3ef3f

codex-rs/app-server/src/request_processors/config_processor.rs

@@ -52,6 +52,7 @@ use std::path::PathBuf;
 const SUPPORTED_EXPERIMENTAL_FEATURE_ENABLEMENT: &[&str] = &[
     "apps",
     "memories",
+    "mentions_v2",
     "plugins",
     "remote_control",
     "tool_search",

codex-rs/app-server/tests/suite/v2/experimental_feature_list.rs

@@ -382,7 +382,7 @@ async fn experimental_feature_enablement_set_rejects_non_allowlisted_feature() -
     );
     assert!(
         error.message.contains(
-            "apps, memories, plugins, remote_control, tool_search, tool_suggest, tool_call_mcp_elicitation"
+            "apps, memories, mentions_v2, plugins, remote_control, tool_search, tool_suggest, tool_call_mcp_elicitation"
         ),
         "{}",
         error.message