feat: screenshot element/region clipping (v0.3.7) (#56)

* feat: screenshot element/region clipping (--clip, --viewport, CSS/@ref)

Add element crop (CSS selector or @ref), region clip (--clip x,y,w,h),
and viewport-only (--viewport) modes to the screenshot command. Uses
Playwright's native locator.screenshot() and page.screenshot({ clip }).
Full page remains the default. Includes 10 new tests covering all modes
and error paths.

* chore: bump version and changelog (v0.3.7)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add screenshot modes to BROWSER.md command reference

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

SKILL.md

@@ -128,6 +128,17 @@ $B viewport 375x812     # iPhone
 $B screenshot /tmp/mobile.png
 $B viewport 1440x900    # Desktop
 $B screenshot /tmp/desktop.png
+
+# Element screenshot (crop to specific element)
+$B screenshot "#hero-banner" /tmp/hero.png
+$B snapshot -i
+$B screenshot @e3 /tmp/button.png
+
+# Region crop
+$B screenshot --clip 0,0,800,600 /tmp/above-fold.png
+
+# Viewport only (no scroll)
+$B screenshot --viewport /tmp/viewport.png
 ```
 
 ### Test file upload
@@ -337,7 +348,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 | `diff <url1> <url2>` | Text diff between pages |
 | `pdf [path]` | Save as PDF |
 | `responsive [prefix]` | Screenshots at mobile (375x812), tablet (768x1024), desktop (1280x720). Saves as {prefix}-mobile.png etc. |
-| `screenshot [path]` | Save screenshot |
+| `screenshot [--viewport] [--clip x,y,w,h] [selector|@ref] [path]` | Save screenshot (supports element crop via CSS/@ref, --clip region, --viewport) |
 
 ### Snapshot
 | Command | Description |