fix(coding-agent): keep Down Arrow in unfinished prompts

[snimu]

Summary

• keep Down Arrow in the chat prompt until the cursor reaches the end before focusing the Agents View entry below it
• replace the old last-visual-line interception with an exact prompt-end boundary
• remove the now-unused protected visual-line wrapper from the base editor

Testing

• npm run check

Linear: ENG-5147

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Note

Low Risk
Localized interactive prompt keybinding change with no auth, data, or API impact; behavior is covered by regression tests referenced in the PR.

Overview
Down Arrow in the chat prompt no longer jumps to the Agents View entry (subagent summary below the prompt) until the cursor is at the end of the prompt text on the last line.

CustomEditor now gates onMoveBelowPrompt on a new isCursorAtEnd() check instead of only being on the last visual (wrapped) line. The first Down press still moves within the editor (e.g. to line end); focus moves below only when the cursor is already at the true end. Empty prompts behave as before: Down can focus below immediately.

The unused isCursorOnLastVisualLine() bridge on the base Editor in pi-tui was removed.

^{Reviewed by Cursor Bugbot for commit 55ae068. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix Down Arrow in CustomEditor to stay in nonempty prompts until cursor reaches end

Previously, pressing Down Arrow on any wrapped last visual line would immediately move focus to the Agents View. Now, CustomEditor.handleInput only calls onMoveBelowPrompt() when the cursor is at the end of the last line, using a new isCursorAtEnd() helper. The old isCursorOnLastVisualLine() method on Editor is removed.

^{Macroscope summarized 55ae068.}