fix(tui): add plain text paste

[40960]

Issue for this PR

Closes #34006

Type of change

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

What does this PR do?

Adds prompt.paste_plain, bound to Ctrl+Alt+V, so clipboard text can be inserted without converting local image or PDF paths into attachments. Existing Ctrl+V behavior is unchanged.

This follows the approach from #34123 by @ametel01, which was closed by automated cleanup.

How did you verify your code works?

• bun test in packages/tui (192 pass, 1 skip)
• bun typecheck in packages/tui
• Repository pre-push typecheck (30 packages)

Screenshots / recordings

Not applicable; this changes TUI clipboard input behavior.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential related PRs found:

1. fix(tui): paste into dialog prompts with ctrl+v #39844 - fix(tui): paste into dialog prompts with ctrl+v

   • Related: Addresses paste functionality in the TUI, likely the existing Ctrl+V behavior that this PR maintains

2. fix: preserve clipboard image paths for path-based MCP tools #36051 - fix: preserve clipboard image paths for path-based MCP tools

   • Related: Deals with clipboard handling and image path attachment behavior, which is the inverse of what this PR addresses (converting paths into attachments vs. pasting plain text)

These are related to clipboard/paste functionality in the TUI, but don't appear to be direct duplicates since PR #40727 specifically adds a new plain text paste feature (Ctrl+Alt+V) while keeping existing behavior intact.

[github-actions]

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