[feature] Composer: add multiline entry and undo/yank recovery #321

Problem and result

In raw TTY mode, both CR and LF submit the draft; Ctrl+J therefore sends it instead of inserting a newline. Editing or killing a long pasted instruction also has no undo or yank recovery. Users must reconstruct a draft or split one task into multiple messages.

Add deliberate newline insertion and bounded recovery to the existing composer. Keep the current single-row rendering, with the existing newline marker and a small line-count hint; a full multiline viewport is outside this slice.

Acceptance checks

• Ctrl+J inserts LF and Enter/CR submits. Document the terminal ambiguity and provide an explicit LF-as-submit compatibility setting; keep non-TTY line input unchanged.
• Ctrl+_ undoes bounded edit transactions. A bracketed paste is one transaction, and typing coalesces predictably. Ctrl+Y restores the last killed text. Cursor position is restored with undo.
• Submission, clearing, and input-owner changes end the recovery scope. Undo cannot restore a previously sent task or change an already reviewed shell attachment.
• New keys operate only while the composer owns input. They do not intercept PTY input, password fields, approval prompts, or shell review, and they preserve streaming typeahead and queued draft edits.
• Focused key/buffer/render fixtures cover Unicode, pasted newlines, resizing, clear/submit boundaries, and combinations with history and slash selection. No model request is needed for editing.

Landing boundary

Coordinate with PR #309, which preserves exact shell review and queue authority. No external editor integration, terminal viewport rewrite, or persistent undo history in this issue.

Evidence and planning

Package: Input and discovery. Priority: P1. Rough engineering estimate: 1–2 days, including focused fixtures; this is a planning estimate, not a measured benchmark.

Reviewed against v4.20.0 at 4f1717bc120917e1b04d0baa939855417ef866a4: src/ui/keys.ts, src/ui/input_line.ts, src/ui/input_render.ts, src/commands/chat.ts.

Comparator: Claude Code interactive mode documents newline entry and undo; Hermes CLI documents multiline input. Reuse Aether’s existing input ownership rather than importing a new terminal framework.

Created from the LOOP-19 review of the README, all 23 open issues, four open PRs, and recent shipped work on 2026-10-09. These are proposed changes; the acceptance checks below have not been executed.