ci: add live test suite workflow

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Types of changes

What types of changes does your code introduce to OpenZeppelin Midnight Contracts?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Split out of #673 per review: the live CI workflow is large and complex, so it
lands on its own instead of alongside the test harness.

Stacked PR. Based on test/multisig-live-harness (#673), so the diff here
is just live.yml and its "Live tests in CI" CONTRIBUTING subsection. Once
#673 merges to main, this retargets to main automatically.

Adds .github/workflows/live.yml, which runs the live suite separately from
the required PR checks:

• Nightly on main (regression net; a failure opens/comments a live-nightly tracking issue that auto-closes on the next green run).
• On demand via workflow_dispatch, optionally scoped with category / filter inputs.
• On a PR via the live-tests label.
• A plan job reads the live-ready categories from the runner and fans out one job per category, each with its own stack and 6-hour job budget.
• Never a required check.

PR Checklist

• I have read the Contributing Guide
• I have added tests that prove my fix is effective or that my feature works. N/A: CI workflow.
• I have added documentation for new methods or changes to existing method behavior.
• CI Workflows Are Passing

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/live-test-workflow

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