Publish standalone SWE-bench Pro E2B integration

[mattteufel-e2b]

Summary

• publishes the standalone SWE-bench Pro E2B builder, immutable build ledger, runner, and grading integration
• keeps Harbor as an optional thin consumer rather than the owner of environment construction
• adds public-tree, secret, packaging, and multi-version CI gates

Validation

• 99 tests on Python 3.10, 3.11, 3.12, and 3.13
• 99 tests from the extracted source distribution
• Ruff lint and format checks
• actionlint, build, Twine, public-tree policy, and gitleaks history scan

This PR is intentionally left unmerged for maintainer review.