feat: add agent status command

[vincentdji]

Summary

• add flora-agent status for service, health, and local sync-queue visibility
• report component health and per-state file counts without initializing a missing database
• add a read-only SQLite opening path so status inspection never runs schema migrations or alters resumable upload state

Why

Operators need a single local command to inspect whether the agent is running and whether uploads are queued or failing. Reusing normal store initialization for this would run migrations, so the status path now opens the existing database in read-only mode.

Validation

• go test ./...