Added reload

Add /reload fresh to rebuild from the file alone

Plain /reload rebuilds the known set from KNOWN_MACS_FILE unioned with
the database, so allow decisions made from the buttons are kept. That is
the right default: silently discarding them would make "Allow
permanently" mean only "until the next reload".

/reload fresh empties the database first, making the file the sole
source of truth. It reports how many stored allows were discarded, and
an unrecognised argument returns usage rather than falling back to
either behaviour, since the two differ in whether data is destroyed.

Slash command arguments now travel on Command.Arg, decoded from the
"text" field of the slash_commands payload.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QSwZJ6jnJ8Gs8mLLkx2mHY