[Bug]: label-sync should not fail when a PR has no linked issue

[coisa]

Summary

The label-sync workflow can fail when a pull request does not reference any linked issue in its title or body.

Current Behavior

The composite action .github/actions/label-sync/copy-linked-issue-labels parses the pull request title/body with a grep pipeline inside command substitution while running under set -euo pipefail.

When no linked issue keyword such as closes #123 or fixes #123 is present, grep exits with status 1 before the script reaches the explicit fallback branch that should print No linked issue was found... and exit successfully.

That makes the workflow fail even though the absence of a linked issue should be treated as a no-op.

Failing Surface

• .github/workflows/label-sync.yml
• .github/actions/label-sync/copy-linked-issue-labels/run.sh

Observed failure:

Prepare all required actions
Run ./.github/actions/label-sync/copy-linked-issue-labels
Run /home/runner/work/dev-tools/dev-tools/./.github/actions/label-sync/copy-linked-issue-labels/run.sh
/home/runner/work/dev-tools/dev-tools/./.github/actions/label-sync/copy-linked-issue-labels/run.sh
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
  GH_TOKEN: ***

Expected Behavior

• If the PR body/title does not reference any issue, the action should log that no linked issue was found and exit with status 0.
• The workflow should only attempt label propagation when a linked issue number is actually present.

Acceptance Criteria

• copy-linked-issue-labels no longer fails when the PR has no linked issue reference.
• The action still copies labels correctly when an issue reference exists.
• A regression test or equivalent focused verification covers the no-linked-issue path.

[coisa]

Reopening because the merged fix in #158 did not actually resolve the failure path for pull requests without linked issues. I am reverting that change and preparing a corrected fix.