docs: Added documentation for the new command passthrough support in the REPL

REPL.md

@@ -350,8 +350,39 @@ The `.exit` command is used to move between modes in the Coyote REPL.
 | `.exit session` | Exit the active session |
 | `.exit agent`   | Exit the active agent   |
 | `.exit rag`     | Exit the active RAG     |
-| `.exit`         | Exit the Coyote REPL      |
+| `.exit`         | Exit the Coyote REPL    |
 
 ## `.help` - Show the help guide
 Just like with any shell or REPL, you sometimes need a little help and want to know what commands are available to you.
 That's when you use the `.help` command.
+
+## `!<command>` - Run an arbitrary shell command
+
+Prefix any line with `!` to run the rest of the line as a shell command instead of sending it to the LLM. Output streams
+straight to your terminal in real time, and you don't spend any tokens and no output is sent to the LLM.
+
+![command-passthrough](./images/repl/command-passthrough.png)
+
+Particularly useful inside [`coyote --sandbox`](Sandboxes) where you need to modify the sandbox itself without running 
+`sbx exec <name> -- <command>`; e.g., `!apt-get install httpie`, `!git pull`, `!cargo build`,
+or anything else you'd normally run in a terminal, without leaving the REPL.
+
+### How it works
+
+- The line is passed to your shell as `<SHELL> -c "<your command>"`. Note that Coyote does _not_ inspect or sanitize the 
+  command, but rather the shell parses it. So pipes (`!ls | grep yaml`), redirects (`!cat > /tmp/out`), env-var 
+  expansion (`!echo "$HOME"`), globbing (`!ls *.toml`), command substitution (`!grep ERROR $(ls *.log)`), and inline 
+  env vars (`!FOO=bar cmd`) all work exactly like they do in a normal terminal.
+- The `!` must be the **first** character of the line. Lines like ` !ls` (leading whitespace), `echo !foo` (inline `!`),
+  or `! ` (just whitespace after) are not recognized as shell pass-through; The first two go to the LLM, the last
+  prints a usage hint.
+- Press `Ctrl-C` at any time to interrupt a long-running command. `SIGINT` propagates to the child shell; the command
+  stops, `[exit 130]` prints, and you return to the prompt.
+- If the command exits non-zero, `[exit N]` is printed. Successful commands return to the prompt silently.
+
+### Limitations
+
+- `!cd /tmp` does _not_ persist. Each `!cmd` runs in a fresh child shell, so subsequent `!pwd` will show the REPL's
+  original working directory, not `/tmp`.
+- The command is _not_ written to your shell's history file (`~/.bash_history`, `~/.zsh_history`). It is, however, saved 
+  in the REPL's reedline history (use up-arrow to recall).