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/connection.rs

@@ -412,6 +412,7 @@ pub struct DaemonOptions<'a> {
     pub proxy_password: Option<&'a str>,
     pub ignore_https_errors: bool,
     pub allow_file_access: bool,
+    pub hide_scrollbars: bool,
     pub profile: Option<&'a str>,
     pub state: Option<&'a str>,
     pub provider: Option<&'a str>,
@@ -476,6 +477,10 @@ fn apply_daemon_env(cmd: &mut Command, session: &str, opts: &DaemonOptions) {
     if opts.allow_file_access {
         cmd.env("AGENT_BROWSER_ALLOW_FILE_ACCESS", "1");
     }
+    cmd.env(
+        "AGENT_BROWSER_HIDE_SCROLLBARS",
+        if opts.hide_scrollbars { "1" } else { "0" },
+    );
     if let Some(prof) = opts.profile {
         cmd.env("AGENT_BROWSER_PROFILE", prof);
     }