feat: "smart" by default

[jqnatividad]

Now that we have several "smart" commands, create a new envvar QSV_STATSCACHE_MODE environment variable. It has three settings:

• auto - use the stats cache if it's valid (the stats-jsonl file exists and is current) - default.
• force - if the cache does not exist, create it by running stats.
• none - do not use the stats cache even if it exists.

This ensures all smart-capable commands are smart by default.