Fixes a bug reported on Discord with relative links inside a
repository's subdir would resolve incorrectly since we were naively
"absoluting" the link destination.

Now, we resolve it against the current (parent) directory. For example,
if lol/x.md has a link

[foo](./some.png) => /lol/some.png (instead of just /some.png)

thanks @cinny.bun.how on bluesky.

also setup nix devshell to configure TANGLED_OAUTH_JWK in a shellhook
for seamless local oauth dev.

[preview](https://cdn.discordapp.com/attachments/1361968270516949092/1371809102023622699/image.png?ex=68247c48&is=68232ac8&hm=d743ac41cf52bdee418f3260a99860fad4ad11d2c105954e663dabf2735e18f3&)

the pull-id was extracted out of transaction