docs(specs): build/compute contract before push — Phase 2B harvest

Establishes the surface that turns CI/CD prediction from runtime-fragile
into a content-addressed contract computed on the dev laptop before push.
Companion to existing Phase 2A (build attestation primitives) and to
rakia/jenkins iterative-improvement specs.

Captures the 12-concern matrix a perfect CI/CD predictor must model:
1. Source-input identity            7. Live target state (DeployAttestation)
2. Build-process identity           8. Build-result history (BuildAttestation)
3. Toolchain identity               9. Cross-pipeline artifact deps
4. Upstream-artifact identity      10. Trigger provenance
5. Registry presence (rakia)       11. Concurrency / abort semantics
6. Deploy as graph node            12. Failure attribution (ValidationAttestation)

Phase 2B scope: brit plan attestation-aware verdicts (skip/build/deploy/
cold-start), brit verify holistic predictor (validation taxonomy),
pre-push hook integration, golden snapshot tests, LLM-first cost asymmetry.

Originating incident: orchestrator-build GitoxideLabs#727/GitoxideLabs#728 over-build (storageClass
commit edfe5c57) — manifest-only change triggered ~75min full rebuild matrix.

Cross-references:
- docs/specs/2026-04-12-brit-design.md (master)
- docs/plans/phases/phase-2a-build-attestation-primitives.md (substrate)
- elohim/rakia/docs/specs/2026-04-27-rakia-as-brit-attestation-executor-design.md (companion)
- genesis/docs/superpowers/specs/2026-04-27-jenkins-as-brit-attestation-producer-design.md (companion)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Mbd06b