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

README.md

@@ -74,7 +74,7 @@ agent-browser scroll <dir> [px]       # Scroll (up/down/left/right)
 agent-browser scrollintoview <sel>    # Scroll element into view (alias: scrollinto)
 agent-browser drag <src> <tgt>        # Drag and drop
 agent-browser upload <sel> <files>    # Upload files
-agent-browser screenshot [path]       # Take screenshot (--full for full page, base64 png to stdout if no path)
+agent-browser screenshot [path]       # Take screenshot (--full for full page, saves to a temporary directory if no path)
 agent-browser pdf <path>              # Save as PDF
 agent-browser snapshot                # Accessibility tree with refs (best for AI)
 agent-browser eval <js>               # Run JavaScript