[BUG] Option+K (Alt+K) overwrites context instead of adding multiple references in VS Code extension

Description

In the VS Code extension, using Option+K (Mac) / Alt+K (Windows/Linux) to add file context now overwrites the previous context instead of adding multiple references.

Previous Behavior

• Option+K inserted an @file:line reference directly into the chat input field as text
• Multiple uses of Option+K would add multiple @ references to the input
• Users could easily add context from multiple files

Current Behavior

• Option+K adds context to a panel below the chat input field
• Using Option+K again replaces the previous context instead of adding to it
• Users cannot add multiple file/selection contexts using this shortcut

Steps to Reproduce

1. Open VS Code with Claude Code extension
2. Open File A, select some code, press Option+K
3. Context from File A appears in the context panel
4. Open File B, select some code, press Option+K
5. Expected: Both File A and File B contexts are shown
6. Actual: Only File B context is shown (File A is overwritten)

Impact

This is a significant regression for workflows that require referencing multiple files:

• Comparing two files
• Asking about relationships between components and their types
• Refactoring code that spans multiple files

Workaround

• Manually type @file1 @file2 in the chat input
• Drag and drop files (limited functionality per #3967)

Environment

• VS Code: 1.108.0
• Claude Code Extension: Latest version
• OS: macOS

Related Issues

• #3967 (multiple files context via drag-and-drop)