docs: use single quotes in all bash examples to prevent shell expansion

All `-e` arguments in bash examples now use single quotes instead of double
quotes. This prevents bash from interpreting `$`, backticks, and backslashes
inside JS code — a major pain point for agents that construct playwriter
commands programmatically.

Changes across skill docs, README, and website copies:

- **Single quotes for one-liners**: `playwriter -s 1 -e 'await page.goto("...")'`
- **Heredoc preferred for multiline**: `<<'EOF'` disables all bash expansion
- **Quoting rules summary**: explains when to use single quotes vs heredoc vs `$'...'`
- **Rewrote mistake #6**: now explains *why* double quotes break (bash `$` expansion
  silently corrupts regex like `/\$[\d.]+/` and template literals)
- **Added locator-scoped snapshot section**: shows `snapshot({ locator: page.locator("main") })`
  which dramatically reduces output size from ~150 lines to ~20 lines by snapshotting
  only a subtree instead of the full page
- **Added single-quote tip** to README quick start and SKILL.md stub

The locator-scoped snapshot feature already existed but wasn't documented.
Discovered during real-world Cloudflare dashboard automation where full page
snapshots were dominated by 60+ sidebar navigation links.

skills/playwriter/SKILL.md

@@ -25,7 +25,9 @@ This outputs the complete documentation including:
 
 ```bash
 playwriter session new
-playwriter -s 1 -e "await page.goto('https://example.com')"
+playwriter -s 1 -e 'await page.goto("https://example.com")'
 ```
 
+**Always use single quotes** for the `-e` argument. Single quotes prevent bash from interpreting `$`, backticks, and backslashes inside your JS code. Use double quotes or backtick template literals for strings inside the JS.
+
 If `playwriter` is not found, use `npx playwriter@latest` or `bunx playwriter@latest`.