feat: Add browserless.io as a browser provider (#502)

* feat: Add browserless as a hosted option + boolean env-parsing utility

* Add ensureDomainFilter, sanitizeExistingPage and move parseBooleanParam

* Add docs in relevant places, fix utils, rename of API env var

* Update readme

* Fix env variable name in readme

* Cleanup session stop urls when errors happen

* Fix browserlessStopUrl not being assigned in happy path

cli/src/output.rs

@@ -2484,7 +2484,7 @@ Options:
                              e.g., --proxy-bypass "localhost,*.internal.com"
   --ignore-https-errors      Ignore HTTPS certificate errors
   --allow-file-access        Allow file:// URLs to access local files (Chromium only)
-  -p, --provider <name>      Browser provider: ios, browserbase, kernel, browseruse
+  -p, --provider <name>      Browser provider: ios, browserbase, kernel, browseruse, browserless
   --device <name>            iOS device name (e.g., "iPhone 15 Pro")
   --json                     JSON output
   --full, -f                 Full page screenshot
@@ -2538,7 +2538,7 @@ Environment:
   AGENT_BROWSER_ANNOTATE         Annotated screenshot with numbered labels and legend
   AGENT_BROWSER_DEBUG            Debug output
   AGENT_BROWSER_IGNORE_HTTPS_ERRORS Ignore HTTPS certificate errors
-  AGENT_BROWSER_PROVIDER         Browser provider (ios, browserbase, kernel, browseruse)
+  AGENT_BROWSER_PROVIDER         Browser provider (ios, browserbase, kernel, browseruse, browserless)
   AGENT_BROWSER_AUTO_CONNECT     Auto-discover and connect to running Chrome
   AGENT_BROWSER_ALLOW_FILE_ACCESS Allow file:// URLs to access local files
   AGENT_BROWSER_COLOR_SCHEME     Color scheme preference (dark, light, no-preference)