History: read-only open and last_activity_ms #16

pet-life needs to know when activity last happened, which means reading signald's SQLite. Duplicating the schema in a second crate would be a coupling that breaks silently.

Read-only matters: the normal open prunes and migrates, and a menu-bar app polling every five seconds must not mutate the daemon's store.

Done when:

• History::open_read_only opens without pruning, migrating or creating.
• History::last_activity_ms returns the most recent KeysPerMin or CommitsToday row with a value above zero, or None.
• SessionSeconds and the hardware signals are excluded: a shell sitting open is not attention, and a sleeping machine still reports a battery percentage.

Part of the menubar-pet stack (docs/superpowers/specs/2026-09-04-menubar-pet-design.md).