[Feature] Mouse click-to-position cursor in TUI input editor

Summary

Support mouse click to reposition the cursor within the TUI prompt input editor. Currently, the only way to navigate within a long prompt is arrow keys and keyboard shortcuts (Option+←/→, Ctrl+A/E). Clicking in the input box does nothing.

Motivation

Modern terminal emulators like Warp, iTerm2, and Kitty all support mouse reporting. The prompt input editor should leverage this to allow click-to-position, which is the expected behavior for any text editing surface in 2026. This is especially painful when editing long or multi-line prompts.

Prior art

This was previously requested in #1030 (item 1: "No Mouse Cursor Placement") but that issue was auto-closed as stale. The feature was never implemented.

Proposed behavior

• Clicking within the input editor text positions the cursor at the clicked character offset
• Works in terminals that support mouse reporting (most modern terminals)
• Can be gated behind a setting (enableMouseSupport) if there are compatibility concerns

Environment

• Claude Code v2.1.80
• macOS 26.4.0 (Apple Silicon)
• Warp terminal