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/SKILL.md

@@ -243,6 +243,9 @@ agent-browser screenshot --full full.png        # full scroll height
 agent-browser screenshot --annotate map.png     # numbered labels + legend keyed to snapshot refs
 ```
 
+Headless Chromium screenshots hide native scrollbars for consistent image output.
+Pass `--hide-scrollbars false` when launching to keep native scrollbars visible.
+
 `--annotate` is designed for multimodal models: each label `[N]` maps to ref `@eN`.
 
 ### Handle multiple pages via tabs