fix(logs): say why a sync set turned an automation off, not just "Automation updated: turned off." #102

Follow-up to #38.

Problem #

When the owner enables a sync set, Airglow offers to turn off the follow-mirror automations the set makes redundant. That deactivation writes the same marker as a plain toggle: "Automation updated: turned off." Weeks later, the automation's log shows it was turned off with no reason and no link to the sync set that replaced it, so the owner can't tell why it stopped or whether turning it back on would double up with the sync.

Evidence #

• lib/sync/redundant-automations.ts:121: the deactivation calls logAutomationUpdate, which writes the generic marker.
• lib/automations/update-log.ts:67-73: the marker message, which has no source or reason.

Expected #

The marker for this deactivation says why, for example "Turned off: a sync set now mirrors these follows." A plain owner toggle keeps the current wording.

Acceptance criteria #

• The sync-set deactivation marker names the reason.
• lib/sync/redundant-automations.test.ts checks the new message.

Out of scope #

Other system rows, such as session-loss and rate-limit disables (see #91).