Fix env apply dropping prompt provenance:

- stamp_apply_provenance is now read-modify-write, preserving an
  env-tip commit's own ai_metadata/test_metrics on fast-forward apply
  (previously force-clobbered with a fresh ai_metadata=None record).
- --patch (squash) apply folds the squashed env commits' prompts (as a
  list, never concatenated), summed test metrics, subjects, and the
  context-branch tip onto the applied commit's env_provenance, so the
  driving prompt stays tracked on the parent branch and survives gc.
- pr.rs compute_aggregates scores folded prompts individually; merge/FF
  leave the fold empty so there is no double count.

Tests: fold collection, stamp RMW preservation, folded-prompt scoring
(+ no-double-count), and an end-to-end --patch fold integration test.