Modernize deps/CI and add model quality + artifact refresh workflow

[5pence5]

Summary

• modernize packaging/tooling around pyproject.toml + uv.lock + Python 3.11 policy
• enforce zero-bypass test policy via skip-audit and CI checks
• add deterministic bootstrap + packaging smoke checks (ci/check_dist_contents.py)
• add CI-enforced contract model quality gate with committed baseline metrics fixture
• add scripts/reexport_contract_model.py for controlled model tag re-export + quality verification
• refresh bundled lexnlp/extract/en/date_model.pickle on current sklearn runtime to reduce legacy warnings from that artifact
• update AGENTS.md, MIGRATION_RUNBOOK.md, and notes.md with operational guidance

CI / Validation

• base suite: 497 passed, 11 skipped
• Stanford suite: 11 passed
• combined demonstrated pass count: 508/508 across base + Stanford runs
• skip audit: OK (markers=11, allowlisted=11, annotated_new=0)
• packaging: uv build + dist-content audit + wheel smoke install
• model quality gate: baseline/candidate checks pass using fixed fixture + baseline metrics

Policy

• no tests were skipped/disabled to make runs green
• skip/xfail bypass markers remain audited and controlled