fix: resolve AI context review workflow CI failures

[udhaykumarbala]

Summary

• Shell quoting fix — replaced $'\n' string concatenation with an add() helper function to avoid shell misinterpretation of grep pipes inside $() subshells (caused Permission denied on CI)
• Contract grep guard — extracted grep results into variables and added if [ -f docs/ai-context.md ] check
• Missing labels — added gh label create with || true before issue creation so it doesn't fail on first run
• Body file — switched from --body "$BODY" to --body-file /tmp/report.md to avoid escaping issues with large markdown

Test plan

• Trigger workflow manually via Actions tab and verify issue is created successfully

[vercel]

@udhaykumarbala is attempting to deploy a commit to the 0g-Frontend Team on Vercel.

A member of the Team first needs to authorize it.