Truncate PR title when searching for existing issue

[awly]

https://github.com/tailscale/ToBeReviewedBot/blob/9d3965af0d3c351cb7a61ef8cf3b69b618fa076d/main.go#L74 creates an API query including the PR title. We need to

1. escape the title in the query to avoid producing fake results
2. truncate the title to avoid hitting query size limits https://docs.github.com/en/rest/search/search?apiVersion=2022-11-28#limitations-on-query-length