fix(ci): skip labeler for fork pull requests

[luoyanglang]

Summary

Issue: CI false-red on fork pull requests in Auto Label PRs
Type: Bug Fix
Severity: Low

This PR avoids a false-red CI failure in fork PRs by skipping the label job when the pull request head repo is a fork. The existing size-label job remains unchanged.

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Root Cause

actions/labeler@v6 needs write permission to add labels to pull requests, but fork PRs triggered via pull_request do not get a write-capable GITHUB_TOKEN in this context.

Current workflow:

• .github/workflows/auto-label.yml:12
• label always runs on pull_request
• for fork PRs, the action fails with Resource not accessible by integration

That means fork-based contribution PRs can go red even when no functional checks are failing.

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Fix Description

Changed files:

• .github/workflows/auto-label.yml:12 - skip the label job when github.event.pull_request.head.repo.fork is true

Why this approach:

• It is the smaller and safer change.
• It preserves current behavior for same-repo PRs.
• It keeps size-label unchanged, so PR sizing still works.

If you would prefer to preserve auto-labeling for fork PRs as well, I’m happy to switch this to a pull_request_target-based approach or another workflow shape you prefer.

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Test Results

Test	Description	Status
Diff inspection	Verified only the label job condition changed	PASS
Existing workflow behavior review	Confirmed size-label remains untouched	PASS

I did not run actionlint locally in this environment.

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Disprove Analysis

Is this already fixed elsewhere?

• I checked the current workflow configuration in main; the label job still runs unconditionally on pull_request.
• The recent failure on cft0808/edict#280 reproduces the problem path directly.

Impact scope

• Same-repo PRs still run the label job as before.
• Fork PRs skip only the failing label step.
• size-label still runs for both cases.

Edge cases

• This does not change repo labeling rules or label matching config.
• This only avoids the permission failure path for fork PRs.

Known limitation

• Fork PRs will not receive actions/labeler labels under this conservative fix.
• If maintaining those labels for forks is important, a different workflow shape would be more appropriate.

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Checklist

• Change is scoped to a single workflow file
• No functional runtime code changed
• Existing same-repo behavior remains intact
• Leaves room for a different maintainer-preferred workflow approach

Found while investigating the failing label check on a fork PR. Happy to adjust if you prefer a different workflow strategy.