feat(e3): unify review packet construction paths

[agustif]

Summary

Implements Epic #209 and sub-issues #219 #220 #221.

This refactor removes duplicated review packet assembly logic in prepare, --run-batches, and --external-start by routing all three through the shared coordinator/builder contract.

What Changed

• Added canonical prepare next-command builder in packet layer:
  • desloppify/app/commands/review/packet/build.py
  • new build_prepare_next_command(...)
• Refactored prepare to use shared coordinator packet path:
  • desloppify/app/commands/review/prepare.py
  • now calls resolve_review_packet_context(...) + build_review_packet_payload(...)
  • preserves zero-file hint behavior and raises CommandError with existing guidance
• Refactored run-batches packet preparation to shared coordinator:
  • desloppify/app/commands/review/batch/orchestrator.py
  • _load_or_prepare_packet(...) now uses shared context/builder and explicit PacketValidationError wrapping
  • keeps legacy blind packet path behavior via blind_path_override=_blind_packet_path()
• Refactored external-start packet preparation to shared coordinator:
  • desloppify/app/commands/review/external.py
  • _prepare_packet_snapshot(...) now uses shared context/builder
  • now includes the same config redaction + file-cap policy path as other modes
  • keeps legacy blind packet path behavior via blind_path_override=_blind_packet_path()
• Added regression tests for parity/delegation:
  • desloppify/tests/review/policy/test_packet_mode_parity.py
  • asserts shared packet fields are equal across prepare/batch/external for same inputs
  • asserts all modes delegate packet assembly to shared coordinator builder

Engineering Outcomes

• One canonical packet-construction seam for all review modes.
• Shared policy application (max_files_per_batch, config redaction, retrospective options) now converges through one path.
• Mode-specific behavior remains explicit only in next_command shaping.
• Reduced drift risk and reduced maintenance surface for future review packet changes.

Verification

• Lint:
  • source .venv/bin/activate && ruff check desloppify/app/commands/review/prepare.py desloppify/app/commands/review/batch/orchestrator.py desloppify/app/commands/review/external.py desloppify/tests/review/policy/test_packet_mode_parity.py
• Tests:
  • source .venv/bin/activate && pytest -q desloppify/tests/review/review_commands_cases.py desloppify/tests/review/import_scoring/test_review_external.py desloppify/tests/commands/review/test_review_process_guards_direct.py desloppify/tests/review/policy/test_packet_mode_parity.py
  • Result: 103 passed

Tracking

Parent program: #206