Move the parser guide into the repo and bring it up to date

The guide lived one directory above the repo, so the README links were
broken on GitHub. It also described the old layout: parsers inside
index.php, a copied <style> block, and div-based tables. It now covers
parsers/{name}.php, emailShell(), esc(), real table markup,
case-insensitive headers, and local testing with a sendmail override.

README.md

@@ -176,7 +176,7 @@ See also the [Webex Interact Guide](WEBEX_INTERACT_GUIDE.md).
 
 ### Creating Your Own Parser
 
-A parser is one file in `parsers/` exposing `{name}Parse($rawBody, $headers, $metadata)` that returns `['html' => ..., 'subject' => ...]` or `false`. Build the body from `.section`, `table.metadata`, and `.data-box` blocks, escape free text with `esc()`, and wrap it with `emailShell()` from `parsers/helpers.php` so it picks up the shared design. See the [Parser Development Guide](../PARSER_PROMPT.md) for details.
+A parser is one file in `parsers/` exposing `{name}Parse($rawBody, $headers, $metadata)` that returns `['html' => ..., 'subject' => ...]` or `false`. Build the body from `.section`, `table.metadata`, and `.data-box` blocks, escape free text with `esc()`, and wrap it with `emailShell()` from `parsers/helpers.php` so it picks up the shared design. See the [Parser Development Guide](PARSER_PROMPT.md) for details.
 
 ---
 
@@ -370,7 +370,7 @@ The standard (no parser) email contains:
 - **Plus Addressing**: Use email plus addressing (e.g., `yourname+github%40gmail.com`) to track which services are sending webhooks
 - **Timestamp in Emails**: Check the timestamp in the header to verify webhook timing and debug delivery delays
 - **Standard Format**: Use the standard format (no parser) when you need complete raw data for debugging
-- **Create Parsers**: Build custom parsers for your favorite services - see [Parser Guide](../PARSER_PROMPT.md)
+- **Create Parsers**: Build custom parsers for your favorite services - see [Parser Guide](PARSER_PROMPT.md)
 
 ## 📝 Error Handling