Feature Request: Generate incremental change reports after feedback

## Problem

When a plan/report is reviewed, feedback is sent to the agent and the agent generates a revised report. The next review currently presents the full report again, so the reviewer must manually re-read everything to determine what changed and whether the previous feedback was addressed.

For iterative review workflows, this creates unnecessary review cost and makes it easy to miss the actual delta.

## Proposed behavior

After each feedback iteration, generate a standardized change report against the immediately previous version. The review surface should make the delta the primary view, while keeping the full revised report available on demand.

The change report could include:

- Added, removed, and modified sections/items, with stable anchors.
- A concise summary of what changed.
- Links from each change to the related prior feedback/annotation when it can be matched.
- An explicit “unchanged” indicator or option to hide unchanged content.
- A fallback to a full review when no reliable baseline exists.

## Example workflow

1. Reviewer annotates v1 and sends feedback.
2. Agent produces v2.
3. Plannotator compares v2 with v1 and renders a standardized incremental report.
4. Reviewer verifies the delta and feedback resolution without re-reading all unchanged content.

## Relationship to existing issues

This is related to #1136 (preserve viewed state for unchanged files) and #825 (recall prior-review annotations in a revised plan diff), but is broader and focuses on producing a first-class, standardized post-feedback change report. It should remain useful even when prior annotations cannot be matched.

## Acceptance criteria

- Each feedback iteration can be opened in a delta-first review mode.
- The delta identifies added/removed/modified content at a stable section/item level.
- Unchanged content is collapsed or excluded by default, with a one-click option to show the full report.
- The previous full report remains accessible for context.
- The format is deterministic enough to support repeatable review and future automation.