feat(telemetry): add JSON export writer

[EhabY]

Summary

• add the JSON array telemetry export writer
• preserve the stored snake_case JSONL event shape in the exported array
• write through a unique temporary file before renaming into place

Refs #903.

Stack: 2 / 4. Base: #959. Next: #961.

Review follow-up

• temp file names use randomUUID() to avoid same-process collisions
• writer setup closes file handles if prefix writing fails
• temp cleanup preserves the original export error
• tests assert full stored event shape and empty-array output

Validation

• pnpm test:extension ./test/unit/telemetry/export/range.test.ts ./test/unit/telemetry/export/files.test.ts ./test/unit/telemetry/export/writers.test.ts
• pnpm typecheck
• pnpm lint
• pnpm format:check

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