[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/common/src/lib.rs

@@ -23,3 +23,7 @@ mod sandbox_summary;
 
 #[cfg(feature = "sandbox_summary")]
 pub use sandbox_summary::summarize_sandbox_policy;
+
+mod config_summary;
+
+pub use config_summary::create_config_summary_entries;