chore(cli): save screenshots to tmp dir when no path provided (#247)

* fix(cli): save screenshots to tmp dir when no path provided

Instead of outputting base64 to stdout (which is not useful for most CLI use cases),
screenshots without a path now save to ~/.agent-browser/tmp/screenshots/ with a
generated filename and return the path.

This makes the behavior more ergonomic for AI agents and CLI users alike.

* cleanup

* cleanup

* just revert the cargo.lock version for now

* refactor: extract getAppDir() from getSocketDir()

* docs: improve screenshot help text consistency

docs/src/app/quick-start/page.tsx

@@ -12,8 +12,8 @@ agent-browser snapshot                    # Get accessibility tree with refs
 agent-browser click @e2                   # Click by ref from snapshot
 agent-browser fill @e3 "test@example.com" # Fill by ref
 agent-browser get text @e1                # Get text by ref
-agent-browser screenshot                  # Base64 png to stdout
-agent-browser screenshot page.png         # Save to file
+agent-browser screenshot                  # Save to a temporary directory
+agent-browser screenshot page.png         # Save to a specific path
 agent-browser close`} />
 
         <h2>Traditional selectors</h2>