feat: show attachments (PRs, commits) in issue view

[paymog]

Summary

• Adds an Attachments section to issue view that displays linked PRs, commits, and other attachments
• Each attachment shows its title as a clickable link and source type (e.g., github, githubCommit)

Example output

## Attachments

- [Enable control plane RDS infrastructure in prod](https://github.com/...) _[github]_
- [enable control plane RDS infrastructure in prod](https://github.com/...) _[githubCommit]_

Test plan

• Run linear issue view <issue-with-pr> and verify attachments are shown
• Verified with real issue that has linked PRs and commits
• Tests pass

🤖 Generated with Claude Code

[paymog]

Closing - upstream already has this feature implemented (1b4135b)

[schpet]

@paymog 1b4135b is about 'file attachments' i.e. pdfs and stuff someone might upload to the description or comments– this sounds like its about 'attachemtns' as an api thing (structured data, like links) on an issue right? if so IMO reopening sounds good - have wanted this for a while - see #21

[paymog]

reopened in #111