Programmatic review input surface for agents and existing files

Provenance stamp: This issue was triaged on June 13, 2026 as part of the older-than-two-month GitHub issue triage exercise.

Source issues: #295, #385

## Summary

The review surface should be usable when there is no local uncommitted git diff. Agents and users both need ways to send specific material into the review UI without manufacturing fake changes.

## What the source issues requested

- #295 requested a `submit_review` tool for the OpenCode extension so an agent can submit review material directly instead of relying only on a human-triggered slash command.
- #385 requested a way to review an existing code file in the code review UI without mutating git state or creating a fake diff.

## Triage determination

Both issues are asking for a programmatic review input contract. One comes from the agent side, and one comes from the user side, but the shared requirement is to open review UI over explicit inputs rather than only over discovered VCS changes.

## Acceptance criteria

- A user can open the review UI for a specific existing file or path without mutating git state.
- An agent can submit review material through a structured tool or command path where the runtime supports it.
- Review feedback preserves file path and line references when reviewing existing files.
- The flow distinguishes real diffs, synthetic review inputs, and read-only file review.
- The command or tool behavior is documented for OpenCode and any other supported runtime.
- The old source issues can be closed after this replacement issue carries both input-surface requirements.