[FEATURE] VSCode Extension: Display context usage percentage in UI

Feature Request

Problem

The CLI version of Claude Code supports custom status lines that can display context usage percentage, but the VSCode extension has no visibility into context consumption. Users must manually run /context to check usage.

This matters because Claude's response quality can degrade as context fills up (commonly cited around 40%). Without visibility, users don't know when to run /compact to maintain optimal performance.

Proposed Solution

Add a persistent context usage indicator to the VSCode extension UI, showing:

• Current context usage percentage (e.g., "Context: 30%")
• Visual warning states:
• 🟢 Green: < 35%
• 🟡 Yellow: 35-45% (suggest /compact)
• 🔴 Red: > 45% (strongly recommend /compact)

Bonus Features (nice to have)

• Configurable threshold warnings
• Optional notification/toast when crossing thresholds
• Auto-suggest /compact when hitting warning threshold

Context

• The CLI statusLine feature doesn't work in VSCode extension
• Tool Search (v2.1.7) helps reduce MCP tool overhead, but conversation context still grows
• Power users track the "40% rule" manually - this should be built-in

Environment

• Claude Code: 2.1.7
• VSCode Extension
• macOS