feat: add VS Code Copilot chat provider

[onurgule]

Summary

• Adds VS Code Copilot as a new provider to Code Trail
• Discovers chat sessions from VS Code's workspaceStorage/*/chatSessions/*.json
• Parses Copilot v3 session format (user messages, markdown responses, tool invocations, elicitations)
• Cross-platform support: macOS, Linux, Windows
• Gracefully handles missing Copilot installations (no errors if user never used Copilot)
• Includes teal accent color in the UI for the Copilot provider

Changes

• 36 modified files, 1 new fixture file (+707 lines, -17 lines)
• Core: provider schema, discovery, parsing, indexing, system message rules
• Desktop: bootstrap, settings, UI components, styles
• Tests: discovery, parsing, IPC, integration tests updated

Note on README path

The Copilot directory in the README is shown as .../Code/User/workspaceStorage/*/chatSessions/ because the prefix varies by platform:

Platform	Full path
macOS	~/Library/Application Support/Code/User/workspaceStorage/*/chatSessions/
Linux	~/.config/Code/User/workspaceStorage/*/chatSessions/
Windows	%APPDATA%/Code/User/workspaceStorage/*/chatSessions/

The code auto-detects the correct path per platform.

Test plan

• Unit tests for Copilot discovery (existing + missing directories)
• Unit tests for Copilot session parsing (markdown, tools, elicitations)
• Integration tests updated to include copilot provider
• Manual testing with real VS Code Copilot chat sessions

Screenshot