OMP-2 — Migrate OMP history, reasoning, tools, edits, usage, and timeline parity to ACP #54

Parent epic: #21
Depends on: #53
Parity contract: #52

Objective

Move the read/render/timeline half of existing OMP Native Chat from transcript decoding to ACP while preserving every #52 parity row owned by this issue.

Scope

Migrate and test:

• session history/load replay;
• user/assistant message ordering;
• assistant streaming;
• reasoning/thinking blocks;
• tool start/update/end;
• tool errors/interruption;
• edit/file-change cards;
• model/provider metadata attached to messages;
• token/context usage;
• compaction/reset boundaries;
• timestamps/identity where required by current UI;
• replay/live transition.

Source-of-truth change

Old:

OMP session JSONL
  → transcript decoder/tailer
  → NativeChatMessage projection

New:

OMP ACP load/live updates
  → shared ACP translator
  → shared timeline assembler
  → journal

Do not keep both sources authoritative for the same structured ACP session.

Replay guarantees

Prove:

• history loads once;
• live messages arriving around load do not duplicate replayed rows;
• repeated attach/restart is idempotent;
• abandoned/incomplete provider activity is represented consistently with generic structured-provider rules;
• compaction/history boundaries are not lost.

Parity tests

For representative recorded OMP conversations, compare the old transcript projection with the ACP projection at the semantic level:

• visible conversation order;
• reasoning presence;
• tool/edit meaning;
• usage/model metadata;
• completion state.

Exact internal row formats need not match if ACP is semantically stronger.

Acceptance criteria

• All OMP-0 — Freeze the terminal/transcript Native Chat parity contract #52 read/render parity rows assigned here are green.
• ACP replay + live updates are duplicate-free.
• Reasoning/tools/edits retain at least current fidelity.
• Usage/model/compaction semantics are preserved or improved.
• ACP structured sessions no longer depend on OMP transcript tailing for authoritative history.