Document diff review mode, split pane, and new review MCP tools

[selfcontained]

Summary

Diff-driven docs audit for changes since 0a4abfb6 (PRs #730, #741–#746). Updates in-app docs to cover:

• Diff review mode — Changes tab inline comment form now defaults to "Start a review" (batch draft comments, submit with summary). "Chat" remains in the dropdown. Review mode bar, submit dialog, and the new Reviews tab in the media sidebar are documented.
• Split pane — Terminal and Changes can now be shown side by side via drag-and-drop. New agents docs section.
• New MCP tools — dispatch_review_list_feedback, dispatch_review_resolve, dispatch_review_add_message added to built-in tools list.
• Cleanup — removed duplicate "Reordering agents" section and duplicate dispatch_launch_agent entry in tools list.
• Tips — added ambient tips for diff review and split pane (since: 0.27.0).

Deferred to next run

• Updates section deep-dive (assisted-update flow, release checks, version display) — original next_focus, deferred because the diff was more urgent.

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