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.

skill-data/core/references/commands.md

@@ -103,6 +103,9 @@ agent-browser screenshot --full   # Full page
 agent-browser pdf output.pdf      # Save as PDF
 ```
 
+Headless Chromium screenshots hide native scrollbars for consistent image output.
+Pass `--hide-scrollbars false` when launching to keep native scrollbars visible.
+
 ## Video Recording
 
 ```bash
@@ -309,6 +312,7 @@ agent-browser --headers <json> ...    # HTTP headers scoped to URL's origin
 agent-browser --executable-path <p>   # Custom browser executable
 agent-browser --extension <path> ...  # Load browser extension (repeatable)
 agent-browser --ignore-https-errors   # Ignore SSL certificate errors
+agent-browser --hide-scrollbars false # Keep native scrollbars visible in headless Chromium screenshots
 agent-browser --help                  # Show help (-h)
 agent-browser --version               # Show version (-V)
 agent-browser <command> --help        # Show detailed help for a command
@@ -383,6 +387,7 @@ AGENT_BROWSER_EXECUTABLE_PATH="/path/chrome" # Custom browser path
 AGENT_BROWSER_EXTENSIONS="/ext1,/ext2"       # Comma-separated extension paths
 AGENT_BROWSER_INIT_SCRIPTS="/a.js,/b.js"     # Comma-separated init script paths
 AGENT_BROWSER_ENABLE="react-devtools"        # Comma-separated built-in init script features
+AGENT_BROWSER_HIDE_SCROLLBARS="false"        # Keep native scrollbars visible in headless Chromium screenshots
 AGENT_BROWSER_PROVIDER="browserbase"         # Cloud browser provider
 AGENT_BROWSER_STREAM_PORT="9223"             # Override WebSocket streaming port (default: OS-assigned)
 AGENT_BROWSER_HOME="/path/to/agent-browser"  # Custom install location