chore: use @plugin instead of $plugin for plaintext mentions (#13921)

change plaintext plugin-mentions from `$plugin` to `@plugin`, ensure TUI
can correctly decode these from history.

tested locally, added/updated tests.

codex-rs/core/src/mention_syntax.rs

@@ -0,0 +1,4 @@
+// Default plaintext sigil for tools.
+pub const TOOL_MENTION_SIGIL: char = '$';
+// Plugins use `@` in linked plaintext outside TUI.
+pub const PLUGIN_TEXT_MENTION_SIGIL: char = '@';