Add comprehensive technical research report for rebase-editor internals

[Copilot]

This PR delivers the requested deep-dive analysis of the repository and documents it in agent/research.md with no code changes. The report captures how the editor works end-to-end, including architecture, behavior, edge cases, and project-specific constraints.

• Scope

  • Added a single research artifact: agent/research.md
  • No runtime/source/test logic changed

• Repository deep-dive coverage

  • System architecture and module responsibilities (index.js, main, parser, reducer, terminal, keybindings, file I/O)
  • State model and event flow (parse → reduce → render → serialize)
  • CLI surface and platform-specific behaviors
  • Rebase command support boundaries (pick/reword/edit/squash/fixup/drop/break, handling of update-ref/label/reset/merge, explicit exec limitation)

• Behavioral specifics documented

  • Parser and serializer rules (validation, comment handling, canonical output shape)
  • Reducer semantics (selection model, movement constraints, drop behavior, break insertion, undo/redo stack behavior)
  • Terminal rendering/viewport logic (incremental redraw, width trimming with style markers, resize behavior)
  • Keybinding customization and dynamic help generation

• Engineering findings section

  • Test suite structure and what behavior is covered
  • Notable edge cases and current limitations observed from code/tests

Example excerpt from the delivered report (state flow summary):

1. Parse file to state.
2. Render state.
3. On key press -> map key to action -> reducer(state, action).
4. Render new state.
5. On quit/abort, serialize and write.

Original prompt

read this project in depth, understand how it works deeply, what it does and all its specificities. when that’s done, write a detailed report of your learnings and findings in agent/research.md. Commit that file to a branch called research. Do no code changes. Only research.

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