Use pull_request_target for PR triage to support forks

[Copilot]

The PR triage workflow fails for pull requests from forks because pull_request runs in the fork's context without write permissions to add labels.

Changes

.github/workflows/issue-triage.yml

• Changed trigger from pull_request to pull_request_target
• Updated event name check in issue_number calculation

.github/workflows/reusable-issue-triage.yml

• Updated job condition and environment variable logic to handle pull_request_target event

Context

pull_request_target runs in the base repository's context, providing access to secrets and write permissions while still receiving the PR's metadata via github.event.pull_request. The existing permissions block (issues: write, pull-requests: write) now applies correctly for fork PRs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>PR triage for forks</issue_title>
<issue_description>I think the PR triage workflow needs to use pull_request_target so it works for PRs originating from forks.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes PR triage for forks #188

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