[issue-labeler] Use env var for prediction threshold; default to 0.05

[jeffhandley]

We ran several training runs of the issue-labeler to evaluate different prediction thresholds. With input from several team members and a vote, we are lowering the threshold down to 0.05 to minimize the needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners occurrences.

This PR adds support for configuring the threshold via an ISSUE_LABELER_PREDICTION_THRESHOLD actions env var, applying the 0.05 value as the default for this repo.

Threshold	Matches	Mismatches	No Prediction
1️⃣ 0.05 (test run)	91.91% (42,866)	8.06% (3,760)	0.03% (13)
2️⃣ 0.10 (test run)	91.71% (42,770)	7.81% (3,640)	0.48% (224)
3️⃣ 0.15 (test run)	91.21% (42,520)	7.40% (3,451)	1.39% (646)
4️⃣ 0.20 (test run)	90.40% (42,140)	6.69% (3,117)	2.92% (1,359)
5️⃣ 0.28 (test run)	88.61% (41,307)	5.41% (2,521)	5.98% (2,788)
6️⃣ 0.40 (test run) [current setting]	84.59% (39,431)	3.62% (1,687)	11.79% (5,498)

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[Copilot]

Pull request overview

This PR makes the issue-labeler prediction threshold configurable and lowers the default from 0.40 to 0.05 based on evaluation of multiple training runs. The change aims to minimize occurrences of issues/PRs needing manual area-label assignment.

Changes:

• Makes prediction threshold configurable via repository variable ISSUE_LABELER_PREDICTION_THRESHOLD
• Changes default threshold from 0.40 to 0.05
• Updates three labeler workflow files consistently

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/labeler-train.yml	Updated THRESHOLD to use configurable vars.ISSUE_LABELER_PREDICTION_THRESHOLD with 0.05 default
.github/workflows/labeler-predict-pulls.yml	Updated THRESHOLD to use configurable vars.ISSUE_LABELER_PREDICTION_THRESHOLD with 0.05 default
.github/workflows/labeler-predict-issues.yml	Updated THRESHOLD to use configurable vars.ISSUE_LABELER_PREDICTION_THRESHOLD with 0.05 default