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

[kriszyp]

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.

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🤖 Filed by Claude on behalf of Kris.