Review Surface: add first-class session diff/review UI backed by official OpenCode data

[insign]

Summary

Turn CodeWalk's existing session diff data into a first-class review surface for both mobile and desktop. The data/domain layer already contains evidence of support for session diffs; the missing piece is a coherent, visible, user-facing review workflow.

This issue should not invent a new server contract. It must use official OpenCode-backed diff data and preserve current chat/scroll semantics.

Why this exists

The repository already contains signs that diff information exists and is being consumed internally:

• lib/domain/usecases/get_session_diff.dart
• lib/presentation/utils/diff_parser.dart
• lib/presentation/pages/chat_page/chat_page_file_runtime.dart
• chatProvider.currentSessionDiff

However, CodeWalk still lacks a truly first-class review surface where a user can confidently inspect changes produced by the assistant. This is one of the highest-value UX gaps versus richer OpenCode-based clients.

Required references

• Resulting matrix from Contract Matrix: build OpenCode source-of-truth and capability inventory for CodeWalk #26
• Contract test protections from Contract Tests: enforce ADR-023 and ADR-019 invariants in CI #27 if transport/reconcile behavior is touched
• Official OpenCode session/diff semantics from docs/source

Current repo evidence to inspect

• lib/domain/usecases/get_session_diff.dart
• lib/presentation/utils/diff_parser.dart
• lib/presentation/pages/chat_page/chat_page_file_runtime.dart
• lib/presentation/pages/chat_page/chat_page_file_viewer.dart
• lib/presentation/pages/chat_page/chat_page_workspace_controller.dart
• lib/presentation/pages/chat_page/chat_page_timeline_builder.dart
• lib/presentation/widgets/chat_message/chat_message_tool_part.dart
• lib/presentation/providers/chat_provider.dart

Scope

Build a review surface with two entry modes:

1. Dedicated review surface

A visible Changes / Review surface tied to the current session that shows diff-backed files in a structured way.

2. Inline drill-down

For relevant tool/file-write events in the message timeline, allow the user to expand or drill into the diff for that message/session context without overwhelming the chat surface.

UX requirements

• Mobile: readable, contained, not dependent on giant desktop panes
• Desktop: efficient review workflow with enough space for file list + diff content
• Lazy loading where possible so the timeline does not become heavy
• Clear navigation from diff entry to file viewer / exact file context if supported
• Must not create scroll thrash or break ADR-backed scroll ownership behavior

Deliverables

• A dedicated session diff/review UI
• Inline diff affordance from the timeline where appropriate
• Diff-to-file navigation where feasible
• Clean empty/loading/error states

Acceptance criteria

• Users can discover current session diffs without hidden debug knowledge.
• There is a clear path from assistant-generated file changes to human review.
• The implementation works on compact and expanded layouts.
• Timeline behavior remains stable while opening/closing diff UI.
• The review surface is backed by official session diff data, not locally fabricated diffs.

Non-goals

• No git staging/commit sidebar in this issue.
• No server-side diff generation changes.
• No community-only VCS workflow.

Testing / validation

• Add widget tests for the new review surface states.
• Add integration coverage for fetching and rendering session diffs.
• Run make check.
• Manually verify mobile + desktop review workflows.

Dependencies

• Depends on Contract Matrix: build OpenCode source-of-truth and capability inventory for CodeWalk #26.
• Strongly benefits from Contract Tests: enforce ADR-023 and ADR-019 invariants in CI #27 being in place.

Notes for the implementer

This issue is about reviewability, not about adding more raw data. Prefer a small number of obvious, high-confidence user entry points.