Export session transcript to Markdown

[Ron537]

Summary

Add an "Export transcript to Markdown" action to the right-click menu of a session row. Generates a clean Markdown file capturing the conversation prompts + responses.

Acceptance criteria

• New IPC window.dplex.sessions.exportToMarkdown(providerId, sessionId): Promise<string> returning the rendered Markdown.
• Implementation in main process leverages the provider's existing parsed-session API (e.g. getSessionPrompts / parsed events).
• Renderer wires the action to a "Save Markdown…" file dialog (window.dplex.shell.showSaveDialog or equivalent).
• Markdown layout: # <session title> heading, **You** ... **Copilot** ... sections per turn, code fences for tool calls / results.
• Tested with a real Copilot session and a real Claude Code session.

Files to touch

• src/preload/index.ts (IPC contract)
• src/main/index.ts or new src/main/services/exportSession.ts
• Provider-specific event parsers (already exist) for the prompts list
• src/renderer/src/components/sessions/SessionItem.tsx (menu action)

Effort: 1-2 days.