feat(replay): add guided pull request reconstruction

[fcoury-oai]

Why

A pull-request diff shows the finished change, but not how its author built it. PR Replay lets reviewers absorb a real change by reconstructing its original hunks, step by step, inside an explicitly approved scratch workspace. The original PR branch remains untouched.

What changed

• Start Replay from a real GitHub PR, a local branch and its actual merge base, or a safe, entirely in-memory demo.
• Show every original source-grounded change in a dedicated, focus-aware native pane with syntax and diff highlighting, scrolling, multi-file navigation, movable docks, keyboard and mouse resizing, and a pinned action bar.
• Reconstruct hunks manually or apply each exact original patch as a single safely attributable and undoable editor transaction.
• Pin original source commits and diff anchors; run Git, GitHub, and scratch preparation in bounded background workers with explicit confirmation and recoverable errors.
• Resume verified scratch worktrees, exact review progress, notes, selected changes, and multiple in-progress reviews without replacing or silently adopting workspaces.
• Verify whether the authenticated user is the original PR author; provide a durable local-only outbox for anchored inline comments, PR summaries, and author-only fix proposals.
• Extend the typed editor-owned plugin API while preserving compatibility with existing plugins; document the complete workflow in docs/PR_REPLAY.md.

Review comments and fix proposals are deliberately local only in this milestone. Replay does not create a GitHub pending review, publish comments, submit an approval, edit the original PR branch, start an agent, stage, commit, or push. Portable save/load, explicitly approved GitHub reviews, and agent-assisted author updates will be separate follow-up milestones.

How to Test

1. Run cargo run -p red, press Space R g, and choose Safe in-memory demo. Confirm the dedicated Replay pane opens beside an editable source buffer without creating files, branches, or network requests.
2. With the Replay pane focused, use h/l to navigate changes and j/k to scroll. Confirm original line numbers, language-aware syntax, added/removed diff colors, meaningful change descriptions, clipped long lines, and the pinned action bar remain readable.
3. Press i, manually reconstruct the displayed diff, and validate with Space R v. On another step, press a and then u; confirm exact one-step application and undo without an interrupting confirmation.
4. Use Ctrl-w w and Ctrl-w H/J/K/L to move focus and reposition the pane. Resize with Ctrl-w >/< or Ctrl-w +/-, and drag the divider; confirm focus cues, layout, and syntax highlighting remain correct.
5. Start a real GitHub PR or local-branch replay. Confirm the repository, immutable head, merge base, author, and exact scratch path are shown before creation; use [/] to navigate multi-file changes.
6. Press c for an inline comment, s for a summary, and r for the local outbox. Verify draft editing and confirmed deletion; verify F is available only for the authenticated original PR author. Confirm no GitHub pending review or published comment is created.
7. Restart with cargo run -p red -- --resume; confirm the original verified worktree, selected change, notes, drafts, and progress return. When several reviews exist, confirm the review picker identifies and reopens the chosen review without creating a replacement scratch branch.
8. Run cargo test -p red --lib replay and cargo clippy --all-targets --all-features -- -D warnings. The complete all-feature workspace suite has also been verified.