Add integration tests for VSCode extension UI logic

[nikomatsakis]

Problem

Manual testing of VSCode extension functionality (comments, webviews, commands) is time-consuming and error-prone. Each change requires:

• Building extension
• Installing in VSCode
• Manual clicking through UI flows
• Verifying terminal integration
• Testing edge cases

This slows development and makes regressions likely.

Proposed Solution

Investigate VSCode extension testing approaches:

Option 1: VSCode Extension Test Runner

• Use @vscode/test-electron for automated UI testing
• Can simulate user interactions, verify webview content
• Integrates with existing test infrastructure

Option 2: Headless Testing Framework

• Mock VSCode APIs for unit testing UI logic
• Test webview message passing, command registration
• Faster but less realistic than full integration tests

Option 3: Hybrid Approach

• Unit tests for core logic (comment placement, markdown processing)
• Integration tests for critical user flows (comment replies, action buttons)
• Manual testing only for final verification

Acceptance Criteria

• Research VSCode testing best practices and tooling
• Implement test infrastructure for at least one UI component
• Document testing approach for future development
• Reduce manual testing overhead by 70%+

Context

Currently working on walkthrough system (#23) where manual testing of comment replies, action buttons, and webview interactions is the primary bottleneck.