RepoPact 2.0.0

RepoPact 2.0 — mandatory preflight by default and provenance-typed records.

Breaking changes

• Preflight is mandatory by default. No work begins until a work item is created and
  propagated through the pact; repopact new stamps the marker automatically. Existing
  repos grandfather their pre-2.0 items — adopt/init set a required_from_date epoch,
  doctor migrates on upgrade. (Decision 0021 supersedes 0018.)
• Provenance-typed records (concrete / provisional / inferred, default concrete)
  on work items and evidence runs. adopt now emits provisional records backed by
  inferred evidence — resolving the adoption trilemma in the implementation (Closed
  and Faithful), not merely relaxing it. doctor ratchets provisional → concrete as
  real evidence arrives. L2 enforces admission (P1), completion-requires-concrete (P2),
  and consistency (P3).

Also in this line

• Comparative benchmark suite scaffolding: PactBench (24 pre-registered tasks across six
  security classes + adversarial evidence-fabrication and context-injection), 10 real
  fixtures, a runnable S5 drift harness, and a model-agnostic harness with a subprocess
  RealRunner. Full paper draft + figure mockups under research/.

Migration: run repopact doctor after upgrading to grandfather pre-2.0 work items.
Full suite: 91 tests passing (2 honest skips for open obligations O-4/O-6).