chore: remove the REPL crate/subcommand (#754)

@oai-ragona and I discussed it, and we feel the REPL crate has served
its purpose, so we're going to delete the code and future archaeologists
can find it in Git history.

codex-rs/justfile

@@ -10,10 +10,6 @@ install:
 tui *args:
     cargo run --bin codex -- tui {{args}}
 
-# Run the REPL app
-repl *args:
-    cargo run --bin codex -- repl {{args}}
-
 # Run the Proto app
 proto *args:
     cargo run --bin codex -- proto {{args}}