LTS-5452: one-line key prompt; review follow-ups

Prompt and export on one line, so pasting the block can't feed the export line to read. Robot: per-OS signature check, .gitignore for the CLI binary. Selenium: ignore browserstack-cli.exe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: MihirR-BS

README.md

@@ -18,8 +18,7 @@ The BrowserStack CLI reads your credentials from environment variables. Copy you
 # macOS / Linux (bash or zsh)
 export BROWSERSTACK_USERNAME="<your-username>"
 # Type the key at the prompt: it is not echoed, and it stays out of your shell history
-printf 'BrowserStack access key: '; read -rs BROWSERSTACK_ACCESS_KEY; echo
-export BROWSERSTACK_ACCESS_KEY
+printf 'BrowserStack access key: '; read -rs BROWSERSTACK_ACCESS_KEY; echo; export BROWSERSTACK_ACCESS_KEY
 ```
 
 ```powershell