Add agentic workflow issue-triage (#19057)

This PR attempts to set up an automatic issue triaging tool using Claude
Code as the agent and GitHub Agentics as the framework:
https://githubnext.github.io/gh-aw/start-here/quick-start/

Based on the `safe-outputs` setting, this agent can only label issues or
write replies. Let's see how useful that is 🤷

You can't run it locally since it does not have a manual trigger and
needs a new github issue instead so my plan is to merge it and then
create some super legit issues to see if it works!

## Test plan

```
α dev/tailwindcss (add-workflow-workflows-issue-triage.md-3051) gh aw status
Workflow Status
Name         | Installed | Up-to-date | Status  | Time Remaining
------------ | --------- | ---------- | ------- | --------------
issue-triage | Yes       | Yes        | Unknown | 29d 22h
```

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

Author: philipp-spiess

.gitattributes

@@ -0,0 +1 @@
+.github/workflows/*.lock.yml linguist-generated=true merge=ours