[codex] Add conversation export

[DorianZheng]

What changed

• added a native conversation export flow in Electron main with dialog.showSaveDialog() and a new dune:export:conversation IPC handler
• added runtime/controller support to collect the full agent transcript across paged transcript reads
• added JSON and Markdown exporters that rewrite attachment references into a sibling assets/ folder
• added renderer export controls in the agent panel, including success/error feedback and compact chat support
• added preload/shared bridge contracts and focused tests for the exporter, preload bridge, controller paging, and panel UI
• tightened an existing runtime test mock type and export filename sanitization while validating the feature end to end

Why

Users need a way to save an agent group's full conversation history in either a human-readable format or a machine-readable format without manually copying content out of the app.

User impact

Agents with an active chat panel now expose an Export action with Markdown (.md) and JSON (.json) options. Exports copy referenced local attachments into an assets/ sibling folder and rewrite exported paths to stay relative.

Validation

• pnpm vitest run src/electron/main/export/conversation-exporter.test.ts src/electron/main/runtime/desktop-runtime-controller.test.ts src/electron/preload.test.ts src/renderer/features/agents/components/AgentPanel.test.tsx
• pnpm tsc --noEmit