rename accessibilitySnapshot to snapshot for shorter agent usage

The function is now exposed as `snapshot()` in the executor sandbox scope.
`accessibilitySnapshot` remains as a backward-compatible alias pointing to
the same function, so existing agents and saved commands continue to work.

Updated all code examples across skill.md, README, PLAYWRITER_AGENTS,
docs, tests, and the extension welcome page to use the shorter name.

extension/src/welcome.html

@@ -325,7 +325,7 @@ playwriter session new
 # navigate to a URL
 playwriter -s 1 -e "await page.goto('https://example.com')"
 # get the accessibility tree of the page
-playwriter -s 1 -e "console.log(await accessibilitySnapshot({ page }))"
+playwriter -s 1 -e "console.log(await snapshot({ page }))"
 # click an element by its accessibility reference
 playwriter -s 1 -e "await page.locator('aria-ref=e5').click()"</code></pre>
       </section>