feat(screenshot): hide scrollbars in headless screenshots (cherry-pick b4f2f37)

Cherry-picks upstream agent-browser #1396. Adds a configurable
--hide-scrollbars flag (AGENT_BROWSER_HIDE_SCROLLBARS env, hideScrollbars
config key, default true) that appends Chrome's --hide-scrollbars launch arg
for headless (non-extension) launches so native scrollbars aren't painted into
screenshots. Plumbed through flags.rs, connection.rs, main.rs, native/actions.rs
and native/cdp/chrome.rs; help text in output.rs + skill-data.

Fork adaptation:
- the arg lands in the headless && !has_extensions block, separate from the
  stealth base args — no interaction with anti-detection.
- dropped upstream docs/, agent-browser.schema.json and README hunks (removed
  or rewritten in this fork).

Verified: cargo check --tests passes.

cli/src/commands.rs

@@ -2751,6 +2751,7 @@ mod tests {
             provider: None,
             ignore_https_errors: false,
             allow_file_access: false,
+            hide_scrollbars: true,
             device: None,
             auto_connect: false,
             force_launch: false,
@@ -2766,6 +2767,7 @@ mod tests {
             cli_proxy: false,
             cli_proxy_bypass: false,
             cli_allow_file_access: false,
+            cli_hide_scrollbars: false,
             cli_annotate: false,
             cli_download_path: false,
             cli_headed: false,