fix(opencode): delete-to-line-start getting stuck at line boundary

[fredsh2k]

Issue for this PR

Closes #21663

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

deleteToLineStart() in @opentui/core only deletes when cursor.col > 0. After clearing a line's content, the cursor is at col 0 and further presses do nothing — it never removes the newline to join with the previous line.

This adds an onKeyDown intercept in the prompt: when delete-to-line-start fires and the cursor is at col 0 with row > 0, it calls deleteCharBackward() to remove the newline, joining with the previous line. Repeated presses then continue clearing upward as expected.

How did you verify your code works?

• Typecheck passes (bun typecheck in packages/opencode)
• Manual testing: typed 3+ lines, pressed Ctrl+U repeatedly — each press now clears a line or joins with the previous one

Screenshots / recordings

N/A — keyboard behavior fix, no UI change.

Checklist

• I have tested my changes locally, see screen recording

opencode-cmd-backspace.mov

• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Fix Ctrl+U / delete-to-line-start getting stuck at line boundary doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍