Support threads.count: auto (CPU-based dynamic thread count)

`threads.count` is set at install time based on the CPU count at that moment. If the instance is later resized (more/fewer CPUs), the thread count stays wrong until manually updated.

## Ask

- Accept `threads.count: auto` (or `null`) in the config, meaning Harper reads the available CPU count at startup and sets threads accordingly.
- This means a resize + restart is all that's needed to pick up the new CPU count — no manual config edit.

## Future extension

`auto` could eventually mean dynamic, on-demand thread creation based on load metrics — not in scope for this ticket.

## Acceptance criteria

- `threads.count: auto` (or `null`) works in `harperdb-config.yaml`.
- Harper logs the computed thread count on startup.
- Existing numeric values still work unchanged.
- Documented.

---

🤖 Filed by Claude on behalf of Kris.