Extract codex-utils-plugins crate (#15746)

## Summary
- extract shared plugin path and manifest helpers into
codex-utils-plugins
- update codex-core to consume the utility crate

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/utils/plugins/src/mention_syntax.rs

@@ -0,0 +1,7 @@
+//! Sigils for tool/plugin mentions in plaintext (shared across Codex crates).
+
+/// 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 = '@';