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/native/actions.rs

@@ -197,6 +197,7 @@ fn launch_hash(opts: &LaunchOptions) -> u64 {
     opts.proxy_password.hash(&mut h);
     opts.user_agent.hash(&mut h);
     opts.allow_file_access.hash(&mut h);
+    opts.hide_scrollbars.hash(&mut h);
     h.finish()
 }
 
@@ -1876,11 +1877,24 @@ fn launch_options_from_env() -> LaunchOptions {
             .unwrap_or(false),
         color_scheme: env::var("AGENT_BROWSER_COLOR_SCHEME").ok(),
         download_path: env::var("AGENT_BROWSER_DOWNLOAD_PATH").ok(),
+        hide_scrollbars: hide_scrollbars_from_env(),
         viewport_size: None,
         use_real_keychain: false,
     }
 }
 
+fn hide_scrollbars_from_env() -> bool {
+    env::var("AGENT_BROWSER_HIDE_SCROLLBARS")
+        .map(|v| !matches!(v.to_ascii_lowercase().as_str(), "0" | "false" | "no" | ""))
+        .unwrap_or(true)
+}
+
+fn hide_scrollbars_from_launch_cmd(cmd: &Value) -> bool {
+    cmd.get("hideScrollbars")
+        .and_then(|v| v.as_bool())
+        .unwrap_or_else(hide_scrollbars_from_env)
+}
+
 async fn try_auto_restore_state(state: &mut DaemonState) {
     let session_name = match state.session_name.as_deref() {
         Some(n) if !n.is_empty() => n.to_string(),
@@ -2044,6 +2058,7 @@ async fn handle_launch(cmd: &Value, state: &mut DaemonState) -> Result<Value, St
             .get("downloadPath")
             .and_then(|v| v.as_str())
             .map(String::from),
+        hide_scrollbars: hide_scrollbars_from_launch_cmd(cmd),
         viewport_size: None,
         use_real_keychain: false,
     };
@@ -8646,17 +8661,21 @@ mod tests {
 
     #[test]
     fn test_launch_options_from_env_defaults() {
-        let _guard = EnvGuard::new(&["AGENT_BROWSER_HEADED"]);
+        let guard = EnvGuard::new(&["AGENT_BROWSER_HEADED", "AGENT_BROWSER_HIDE_SCROLLBARS"]);
+        guard.remove("AGENT_BROWSER_HEADED");
+        guard.remove("AGENT_BROWSER_HIDE_SCROLLBARS");
         let opts = launch_options_from_env();
         assert!(opts.headless);
         assert!(opts.args.is_empty());
         assert!(!opts.allow_file_access);
+        assert!(opts.hide_scrollbars);
     }
 
     #[test]
     fn test_launch_options_from_env_headed_flag() {
-        let _guard = EnvGuard::new(&["AGENT_BROWSER_HEADED"]);
-        _guard.set("AGENT_BROWSER_HEADED", "1");
+        let guard = EnvGuard::new(&["AGENT_BROWSER_HEADED", "AGENT_BROWSER_HIDE_SCROLLBARS"]);
+        guard.set("AGENT_BROWSER_HEADED", "1");
+        guard.remove("AGENT_BROWSER_HIDE_SCROLLBARS");
         let opts = launch_options_from_env();
         assert!(
             !opts.headless,
@@ -8664,6 +8683,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_launch_options_from_env_hide_scrollbars_false() {
+        let guard = EnvGuard::new(&["AGENT_BROWSER_HIDE_SCROLLBARS"]);
+        guard.set("AGENT_BROWSER_HIDE_SCROLLBARS", "false");
+        let opts = launch_options_from_env();
+        assert!(!opts.hide_scrollbars);
+    }
+
+    #[test]
+    fn test_launch_cmd_hide_scrollbars_missing_uses_env_default() {
+        let guard = EnvGuard::new(&["AGENT_BROWSER_HIDE_SCROLLBARS"]);
+        guard.set("AGENT_BROWSER_HIDE_SCROLLBARS", "false");
+
+        assert!(!hide_scrollbars_from_launch_cmd(&json!({
+            "action": "launch"
+        })));
+    }
+
+    #[test]
+    fn test_launch_cmd_hide_scrollbars_explicit_overrides_env_default() {
+        let guard = EnvGuard::new(&["AGENT_BROWSER_HIDE_SCROLLBARS"]);
+        guard.set("AGENT_BROWSER_HIDE_SCROLLBARS", "false");
+
+        assert!(hide_scrollbars_from_launch_cmd(&json!({
+            "action": "launch",
+            "hideScrollbars": true
+        })));
+    }
+
     #[test]
     fn test_har_entry_to_json_enriches_request_and_response() {
         // wall_time: 2026-03-15T12:00:00Z = 1_773_576_000