docs(skill/help): document fork.51 features — coordinate click, aliases, stale-sessionId + @url drift checks

SKILL.md + click --help + README now cover what agents could otherwise
only discover by trial:
- coordinate click (click <x> <y> / <x>,<y> / --coords) as a first-class form
- tabs / get-text aliases
- the 'stale sessionId … re-open your target URL' relay error and how to recover
- eval/screenshot/network '@ <url>' stamps as a per-read wrong-tab sanity check
- network requests --clear as the 'start capturing fresh' step

README.md

@@ -178,6 +178,7 @@ agent-browser open https://example.com
 
 # Everything works through your logged-in browser
 agent-browser click "Post"
+agent-browser click 449 320            # …or click a raw viewport coordinate
 agent-browser fill "Title" "Hello World"
 agent-browser screenshot ./page.png
 ```