[feat] add /status slash command (#1873)

- Added a `/status` command, which will be useful when we update the
home screen to print less status.
- Moved `create_config_summary_entries` to common since it's used in a
few places.
- Noticed we inconsistently had periods in slash command descriptions
and just removed them everywhere.
- Noticed the diff description was overflowing so made it shorter.

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -33,8 +33,8 @@ use std::time::Instant;
 
 use crate::event_processor::CodexStatus;
 use crate::event_processor::EventProcessor;
-use crate::event_processor::create_config_summary_entries;
 use crate::event_processor::handle_last_message;
+use codex_common::create_config_summary_entries;
 
 /// This should be configurable. When used in CI, users may not want to impose
 /// a limit so they can see the full transcript.