fix: convert sync I/O to async in session discovery to prevent VS Code UI freeze

[rajbos]

Problem

Two user reports of VS Code becoming completely unresponsive after installing the extension:

• "It just makes VS Code unstable, Git would not start"
• "It's not starting properly and seems to be blocking other extensions as well. It's stuck at loading the token tracker stage."

From the attached screenshot, the extension activates quickly but the status bar is permanently stuck at "AI Fluency: Loading..." while VS Code's UI is frozen (blank splash screen, no editor loaded).

Root Cause

getCopilotSessionFiles() in sessionDiscovery.ts is declared async but performed all filesystem I/O synchronously using fs.existsSync, fs.readdirSync, and fs.statSync. Since JavaScript is single-threaded, synchronous I/O blocks Node.js's entire event loop — including VS Code's UI rendering and all other extension activity.

The scan touches:

• All workspaceStorage/<hash>/chatSessions/ directories (one per workspace — can be 500+)
• globalStorage/github.copilot-chat/ (recursive scan)
• Copilot CLI ~/.copilot/session-state/ subdirectories (one statSync per session)
• OpenCode storage directory (recursive scan)
• Crush and Continue data directories

With 300+ sessions across many workspaces, this froze VS Code for long enough to break Git and other extensions.

Fix

Converted all filesystem operations in getCopilotSessionFiles() and scanDirectoryForSessionFiles() to async equivalents:

• Added private async pathExists(p) helper using fs.promises.access() to replace fs.existsSync()
• fs.readdirSync() → await fs.promises.readdir()
• fs.statSync() → await fs.promises.stat()
• Made scanDirectoryForSessionFiles() async throughout
• Converted the inline OpenCode recursive scan to an async function
• Simplified the Copilot CLI loop: combined existsSync + statSync into a single await fs.promises.stat() call (catches ENOENT instead)

Async I/O yields the event loop between operations, allowing VS Code's UI and other extensions to remain responsive during the filesystem scan.

Closes #493