Add JSON chat history export with full metadata

[mikemikimike]

Summary

• Add /export --json support with a generated JSON filename.
• Treat explicit .json filenames as JSON exports while preserving Markdown exports.
• Preserve the complete Message[] transcript and include session date, provider, model, token count, and session working directory metadata.
• Use atomic writes for explicit JSON export paths.
• Keep the lazy command registry description synchronized with the JSON export behavior.

Tests

• 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
• Full pnpm run test:ava is not clean on Windows because of existing platform-specific failures; all export command tests pass.

Fixes #1309

Issue: #1309

[github-actions]

nc-review: comments — 1 important, 1 nit

@mikemikimike — a few things worth a look, none blocking.

Adds /export --json and .json-extension auto-detection, serialising the full Message[] transcript with session metadata to disk. The implementation is correct: argument parsing, atomic writes for explicit JSON paths, generated names via writeUniqueFile, and path containment all behave as described. Tests cover the new metadata round-trip and the explicit-filename path. One documentation-syncing issue and one minor UX gap are worth raising.

🟠 important · completeness · source/commands/lazy-registry.ts:128

The lazy-registry entry for export still has description: 'Export the chat history to a markdown file'. CLAUDE.md states: 'Keep the description in sync with the module's own description field — they should match.' This description surfaces in /help and the slash-command picker, so users will see the stale text after this PR ships. Update it to 'Export the chat history to a markdown or JSON file' to match the module.

⚪ nit · design · source/commands/export.tsx:104

/export --json <file.json> silently ignores <file.json> — userProvided is false whenever args[0] === '--json', so the second arg never reaches requestedFilename. Not a regression (the previous code never accepted a filename alongside a flag either) but worth a thought: either treat /export --json <file> as user-provided JSON, or reject the extra arg with an explanatory message.

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_{🔴 blocking · 🟠 a reviewer would ask for a change · ⚪ optional}

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[mikemikimike]

/re-review