[Feature] Export chat history to JSON with full tool call and token metadata (/export --json)

[akramcodez]

Description

Extend the /export command to support exporting conversation transcripts as structured JSON (/export --json or /export <filename>.json).

Use Case

Currently, /export only writes a Markdown representation (.md). In the Markdown export, tool execution structures, parameter JSON, image attachments, reasoning blocks, and provider token usage snapshots are flattened or omitted. Developers and researchers benchmarking models or evaluating agents often need the complete, unaltered message history in JSON format for replays, evaluations, or test harness integration.

Proposed Solution

1. In source/commands/export.tsx:
   • Inspect args for --json or a filename ending in .json.
   • When JSON format is requested, serialize the Message[] array with session metadata (session date, provider, model, total tokens, session cwd).
   • Use atomic write to save the JSON export file.
2. Example usage:
   • /export --json -> exports to nanocoder-chat-YYYY-MM-DD.json
   • /export benchmark-run.json

Alternatives Considered

Manually locating the internal .nanocoder-sessions storage files, but users often work with unsaved/ephemeral sessions or want to export directly from the active TUI.

Additional Context

• I have searched existing issues to ensure this is not a duplicate
• This feature aligns with the project's goals (local-first AI assistance)

[mikemikimike]

Implemented in PR #1378: #1378

• Added /export --json with generated JSON filenames and .json auto-detection.
• Added support for /export --json <file.json> while preserving Markdown export behavior.
• Preserved the complete Message[] transcript and session metadata: date, provider, model, total tokens, and session working directory.
• Used atomic writes for explicit JSON paths and synchronized the lazy command description.

Validation:

• pnpm exec ava source/commands/export.spec.tsx (32 passed)
• pnpm run test:format
• pnpm run test:types
• pnpm run test:types:vscode
• pnpm run test:lint
• pnpm run test:knip
• pnpm run test:audit
• pnpm exec changeset status
• All required PR checks passed. The full AVA suite was also run on Windows; its remaining failures are existing platform-specific failures outside the export command, while all export tests pass.