tidy up the pasting behavior with PR/issue URLs

[adboio]

problem

tl;dr pasting a github PR URL auto-converts to a chip, which is usually good, but:

• bad when you're trying to paste in as a skill input for example
• also seems like the general paste chip is taking over sometimes instead of the custom PR one?

context: https://posthog.slack.com/archives/C09G8Q32R6F/p1778086407569999