Move terminal module to terminal-detection crate (#15216)

- Move core/src/terminal.rs and its tests into a standalone
terminal-detection workspace crate.
- Update direct consumers to depend on codex-terminal-detection and
import terminal APIs directly.

---------

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

codex-rs/exec-server/src/server/filesystem.rs

@@ -36,7 +36,7 @@ pub(crate) struct ExecServerFileSystem {
 impl Default for ExecServerFileSystem {
     fn default() -> Self {
         Self {
-            file_system: Arc::new(Environment.get_filesystem()),
+            file_system: Arc::new(Environment::default().get_filesystem()),
         }
     }
 }