Spike: explore & decide how vettd-cli performs scheduled scans

[JTP75]

Description

Explore options for vettd-cli to perform scans on a schedule (daemon process, OS-level scheduler, etc.) in support of automated rescanning, and decide which approach to pursue.

Part of AgenticHighway/vettd#819.

Scope

• Evaluate options: long-running daemon, OS scheduler integration (cron/systemd timer/launchd), external trigger via API, etc.
• Consider cross-platform support, resource footprint, and operational complexity of each option
• Consider how scheduled scans coordinate with the vettd backend (auth, result submission, avoiding duplicate/overlapping scans)

Acceptance Criteria

• Options evaluated with tradeoffs documented
• Recommended approach decided
• Follow-up implementation issue(s) opened reflecting the decision