[FEATURE] Add context window usage visualization to VS Code Copilot UI

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

VS Code Copilot (Claude mode) stops unexpectedly during ongoing coding tasks without any warning or clear indication of the cause. Investigation suggests this is caused by reaching the context window limit. There is no visibility into current context usage, making it impossible for users to anticipate when the agent will stop or manage their context consumption proactively. This lack of visibility is a critical blocker for productive coding workflows.

Proposed Solution

mplement a context window usage indicator in the VS Code Copilot UI that displays:

• Current context usage (tokens/percentage)
• Remaining context budget
• Real-time updates as context accumulates during the session

This should work similarly to the context visualization already available in local Claude mode.

Alternative Solutions

1. Implement automatic context cleanup/summarization when approaching limits
2. Add warning notifications when context usage reaches 70-80% capacity
3. Provide session reset recommendations before context runs out
4. Add a context export feature to save conversation state and start fresh sessions

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

During a typical coding session:

1. User starts working on a feature implementation
2. As the conversation progresses, context accumulates from code reviews, modifications, and discussions
3. Without warning, the agent stops responding
4. User is left confused about what went wrong and unable to continue work
5. With context visualization, user would see context approaching 90%+ and could proactively save their work or start a new session

Additional Context

Local Claude mode already successfully displays context usage metrics. The same implementation pattern should be adapted for the VS Code Copilot extension to provide consistency across different Claude interfaces.