feat: structured plugin parsing (#13711)

#### What

Add structured `@plugin` parsing and TUI support for plugin mentions.

- Core: switch from plain-text `@display_name` parsing to structured
`plugin://...` mentions via `UserInput::Mention` and
`[$...](plugin://...)` links in text, same pattern as apps/skills.
- TUI: add plugin mention popup, autocomplete, and chips when typing
`$`. Load plugin capability summaries and feed them into the composer;
plugin mentions appear alongside skills and apps.
- Generalize mention parsing to a sigil parameter, still defaults to `$`

<img width="797" height="119" alt="image"
src="https://github.com/user-attachments/assets/f0fe2658-d908-4927-9139-73f850805ceb"
/>

Builds on #13510. Currently clients have to build their own `id` via
`plugin@marketplace` and filter plugins to show by `enabled`, but we
will add `id` and `available` as fields returned from `plugin/list`
soon.

####Tests

Added tests, verified locally.

codex-rs/tui/src/mention_codec.rs

@@ -7,6 +7,8 @@ pub(crate) struct LinkedMention {
     pub(crate) path: String,
 }
 
+const TOOL_MENTION_SIGIL: char = '$';
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct DecodedHistoryText {
     pub(crate) text: String,
@@ -31,7 +33,7 @@ pub(crate) fn encode_history_mentions(text: &str, mentions: &[LinkedMention]) ->
     let mut index = 0usize;
 
     while index < bytes.len() {
-        if bytes[index] == b'$' {
+        if bytes[index] == TOOL_MENTION_SIGIL as u8 {
             let name_start = index + 1;
             if let Some(first) = bytes.get(name_start)
                 && is_mention_name_char(*first)
@@ -46,7 +48,7 @@ pub(crate) fn encode_history_mentions(text: &str, mentions: &[LinkedMention]) ->
                 let name = &text[name_start..name_end];
                 if let Some(path) = mentions_by_name.get_mut(name).and_then(VecDeque::pop_front) {
                     out.push('[');
-                    out.push('$');
+                    out.push(TOOL_MENTION_SIGIL);
                     out.push_str(name);
                     out.push_str("](");
                     out.push_str(path);
@@ -75,11 +77,12 @@ pub(crate) fn decode_history_mentions(text: &str) -> DecodedHistoryText {
 
     while index < bytes.len() {
         if bytes[index] == b'['
-            && let Some((name, path, end_index)) = parse_linked_tool_mention(text, bytes, index)
+            && let Some((name, path, end_index)) =
+                parse_linked_tool_mention(text, bytes, index, TOOL_MENTION_SIGIL)
             && !is_common_env_var(name)
             && is_tool_path(path)
         {
-            out.push('$');
+            out.push(TOOL_MENTION_SIGIL);
             out.push_str(name);
             mentions.push(LinkedMention {
                 mention: name.to_string(),
@@ -106,13 +109,14 @@ fn parse_linked_tool_mention<'a>(
     text: &'a str,
     text_bytes: &[u8],
     start: usize,
+    sigil: char,
 ) -> Option<(&'a str, &'a str, usize)> {
-    let dollar_index = start + 1;
-    if text_bytes.get(dollar_index) != Some(&b'$') {
+    let sigil_index = start + 1;
+    if text_bytes.get(sigil_index) != Some(&(sigil as u8)) {
         return None;
     }
 
-    let name_start = dollar_index + 1;
+    let name_start = sigil_index + 1;
     let first_name_byte = text_bytes.get(name_start)?;
     if !is_mention_name_char(*first_name_byte) {
         return None;
@@ -183,6 +187,7 @@ fn is_common_env_var(name: &str) -> bool {
 fn is_tool_path(path: &str) -> bool {
     path.starts_with("app://")
         || path.starts_with("mcp://")
+        || path.starts_with("plugin://")
         || path.starts_with("skill://")
         || path
             .rsplit(['/', '\\'])
@@ -198,9 +203,9 @@ mod tests {
     #[test]
     fn decode_history_mentions_restores_visible_tokens() {
         let decoded = decode_history_mentions(
-            "Use [$figma](app://figma-1) and [$figma](/tmp/figma/SKILL.md).",
+            "Use [$figma](app://figma-1), [$sample](plugin://sample@test), and [$figma](/tmp/figma/SKILL.md).",
         );
-        assert_eq!(decoded.text, "Use $figma and $figma.");
+        assert_eq!(decoded.text, "Use $figma, $sample, and $figma.");
         assert_eq!(
             decoded.mentions,
             vec![
@@ -208,6 +213,10 @@ mod tests {
                     mention: "figma".to_string(),
                     path: "app://figma-1".to_string(),
                 },
+                LinkedMention {
+                    mention: "sample".to_string(),
+                    path: "plugin://sample@test".to_string(),
+                },
                 LinkedMention {
                     mention: "figma".to_string(),
                     path: "/tmp/figma/SKILL.md".to_string(),
@@ -218,7 +227,7 @@ mod tests {
 
     #[test]
     fn encode_history_mentions_links_bound_mentions_in_order() {
-        let text = "$figma then $figma then $other";
+        let text = "$figma then $sample then $figma then $other";
         let encoded = encode_history_mentions(
             text,
             &[
@@ -226,6 +235,10 @@ mod tests {
                     mention: "figma".to_string(),
                     path: "app://figma-app".to_string(),
                 },
+                LinkedMention {
+                    mention: "sample".to_string(),
+                    path: "plugin://sample@test".to_string(),
+                },
                 LinkedMention {
                     mention: "figma".to_string(),
                     path: "/tmp/figma/SKILL.md".to_string(),
@@ -234,7 +247,7 @@ mod tests {
         );
         assert_eq!(
             encoded,
-            "[$figma](app://figma-app) then [$figma](/tmp/figma/SKILL.md) then $other"
+            "[$figma](app://figma-app) then [$sample](plugin://sample@test) then [$figma](/tmp/figma/SKILL.md) then $other"
         );
     }
 }