E3.1: Route all review modes through canonical packet builder

[agustif]

Parent epic: #209
Program: #206

Route prepare, run-batches, and external packet setup through the same coordinator/builder API.

Acceptance

• Single packet-construction implementation remains.
• Entry points only orchestrate control flow.

[agustif]

Implemented on branch feat/e3-review-packet-unification in PR #228 (commit cd91f96).

State update:

• prepare, run-batches, and external-start now delegate packet assembly through build_review_packet_payload.
• Entrypoints now orchestrate only; packet construction logic is centralized.
• Regression coverage added in desloppify/tests/review/policy/test_packet_mode_parity.py to lock delegation behavior.

Validation: review-path suite passed (103 passed).

[agustif]

Completed via implementation PR(s) in this program. Closing as done.