ci(close-prs): surface bun exit code with ::warning:: on failure

[niStee]

Summary

• Wrap the bun script invocation in the close-prs workflow so a non-zero exit code emits a ::warning:: annotation visible in the run log
• Makes the failure mode (GraphQL error, token-scope error, unhandled exception) diagnosable without reading the full log

Motivation

Trialed against fork run #13 of close-prs: the workflow failed with no visible signal in the annotation summary; the root cause was only findable by digging through the raw log. This change surfaces such failures as warnings at the job level.

Testing

• Verified locally that a non-zero bun exit triggers the ::warning:: path (fork run)
• Syntax-checked workflow YAML

[github-actions]

Hey! Your PR title ci(close-prs): surface bun exit code with ::warning:: on failure doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
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Where scope is the package name (e.g., app, desktop, opencode).

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