feat: add --download-path option (#536)

* feat: add --download-path option

Adds a `--download-path` flag (and `AGENT_BROWSER_DOWNLOAD_PATH` env / `downloadPath` config key) to set a default download directory for browser downloads.

Without this, Playwright stores downloads in a temp directory that is deleted when the browser closes. The new option passes through to Playwright's `downloadsPath` on `launch()` and `launchPersistentContext()`.

Fixes #507

* improvements

* fixes

* fixes

cli/src/commands.rs

@@ -1896,8 +1896,10 @@ mod tests {
             cli_proxy_bypass: false,
             cli_allow_file_access: false,
             cli_annotate: false,
+            cli_download_path: false,
             annotate: false,
             color_scheme: None,
+            download_path: None,
         }
     }