Add CI job to report main branch build failures via GitHub issues

[joostjager]

Adds a notify-failure job to the main CI workflow (build.yml) that automatically creates a GitHub issue when the main branch build fails. The job uses inline gh CLI commands (no third-party action dependency) to create or comment on an issue labeled "build failed". It triggers on main branch pushes when at least one job in build.yml has failed.

The job depends on all existing jobs in build.yml and runs after they complete. If any job fails, it either creates a new issue with the "build failed" label or adds a comment to an existing open issue with that label. Each comment includes the workflow run link, event, branch, commit, and committer (as an @-mention). Closing the issue signals the problem is resolved; subsequent failures will create a fresh issue. The first committer whose push triggers a failure is assigned to the issue; subsequent failures that add comments do not reassign.

Note that this only covers jobs in build.yml. Other workflows that run on main pushes (semver.yml, ldk-node-integration.yml) are not covered yet, as GitHub Actions needs cannot cross workflow boundaries. Expanding coverage would require some restructuring (e.g., converting them to reusable workflows or duplicating the notification job).

Permissions are scoped to issues: write only, so the token cannot be used to push code, modify workflows, or access secrets.

Tested on the main branch of my fork by deliberately breaking the build. The action successfully created an issue: joostjager#7

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[joostjager]

For ldk-node-integration.yml it might be simpler to just duplicate the notification job there, since it's a standalone integration test that doesn't share any dependencies with the other CI jobs...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.08%. Comparing base (0b45bfd) to head (3a61ec2).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4418      +/-   ##
==========================================
+ Coverage   86.07%   86.08%   +0.01%     
==========================================
  Files         156      156              
  Lines      103442   103623     +181     
  Branches   103442   103623     +181     
==========================================
+ Hits        89033    89200     +167     
- Misses      11894    11907      +13     
- Partials     2515     2516       +1     

Flag	Coverage Δ
tests	86.08% <ø> (+0.01%)	⬆️

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[TheBlueMatt]

Can we somehow restrict this to only opening new issues?

[joostjager]

I don't think it can be more granular than "issues: write", which also allows updating. That is useful to amend an already existing, unresolved, build issue.

[TheBlueMatt]

If we can use the standard github tooling can we just use gh issue create instead of jayqi/failed-build-issue-action?

[joostjager]

Done