v0.4.0

Added

• base_sha receipt field — the diff base is now PINNED, killing the
  recurring diff_sha256 staleness REWORKs. The binding hash is
  sha256(git diff <base>...HEAD), a 3-dot diff whose base is the merge-base
  of the branch and origin/main. In a high-merge-velocity repo that merge-base
  drifts: an agent stamps against a slightly stale local origin/main while
  the gate fetches a fresher one → a different merge-base → a different hash →
  a spurious REWORK on a content-clean PR. The fix: plumb receipt --write
  (and stamp / new) now record base_sha = the exact merge-base commit and
  compute diff_sha256 as the 2-dot git diff <base_sha>..HEAD (byte-for-byte
  identical to the old 3-dot, but deterministic — it names the base commit
  instead of re-deriving it). The gate verifies against that pinned commit and
  does not re-derive the base from the live origin/main, so concurrent
  merges / a stale local main / the synthetic GitHub merge-ref checkout can no
  longer move the hash.

Security

• Pinned-base ancestry backstop. The gate asserts base_sha is a real
  ancestor of the default branch (git merge-base --is-ancestor). A forged or
  unrelated-history base — which could otherwise hide changes by diffing against
  the wrong commit — is rejected. The receipt still attests the branch's true
  diff off a legitimate fork point.

Compatibility

• Fully back-compatible. A receipt without base_sha (old-format receipts,
  consumers still on @v0.3.0) verifies via the original origin/main-derived
  3-dot path — nothing breaks during rollout. base_sha is optional in the
  schema. Consumer repos can re-pin to @v0.4.0 at their own pace.