feat(usage): split usage analysis view into 3 tabs

Split the Usage Analysis webview into three subtabs:
- 📊 My Activity: Sessions Summary, Interaction Modes, Context References, Multi-Model Usage
- 🔧 Tools & Integrations: Tool Usage, MCP Tools
- 🏗️ Workspace Health: Copilot Customization Files, Missed Potential, Repository Hygiene

Active tab persists across stats refreshes. The Tools tab is
automatically activated when highlighting unknown MCP tools.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rajbos