Incremental re-review of PRs changed since the last review

[Beroni]

Description

When a PR already has a saved review and shows as ⟳ changed since review, re-running the agent should be incremental: hand it the previous review and the commit it covered, and ask it to focus on what changed since.

Motivation

The reviewed index already records the head commit of each review — that's how ⟳ is detected. A full re-review of a large PR after a one-line fix burns tokens re-deriving everything the previous run already found.

Proposed Solution

• New prompt placeholders: {{previous_review}} (body of the last saved review for this PR) and {{reviewed_commit}} (the head commit it covered).
• When the enqueued PR has a mark with an older commit, fill them in; otherwise they render empty.
• The agent template can then instruct: "a previous review and the commit it covered are provided; verify its findings still hold and review only the changes since".
• The saved report should note it was an incremental pass on top of a previous review.

Acceptance Criteria

• {{previous_review}} and {{reviewed_commit}} are available to agent prompts
• Re-reviewing a ⟳ PR fills them from the stored mark and saved review
• First-time reviews behave exactly as today