feat: replace-input-panel-with-usecursor — replace InputPanel cosmetic cursor with useCursor hook

[avoidwork]

Description

Proposes an OpenSpec change to replace the cosmetic cursor character in InputPanel with Ink's useCursor hook for proper terminal cursor positioning. This is the proposal/design phase — implementation tasks follow in subsequent commits.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

OpenSpec artifacts generated: proposal.md, design.md, specs/tui-cursor-positioning/spec.md, tasks.md. Implementation and tests will follow in subsequent commits on this branch.

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied