refactor(cli): deepen live scan lifecycles

[xingkaixin]

Summary

• move adaptive refresh scheduling, pending reruns, and per-agent operation serialization into RefreshCoordinator
• move backfill queue deduplication and sequencing into BackfillCoordinator
• move scan lifecycle transitions and detached snapshots into ScanStatusModel
• move search-index worker ownership, coalesced batches, settlement, and shutdown into SearchIndexJobRunner
• preserve the LiveScanStore external interface while reducing its implementation from 1,382 to 928 lines

Why

LiveScanStore owned four independent state machines through shared mutable fields. Lifecycle changes required coordinated edits across a 1,382-line implementation, and characterization tests reached through private fields to exercise behavior.

The extracted modules are concrete internal modules, not speculative ports. They own their state and return transitions or snapshots to the store, which remains responsible for orchestration and external publication.

Validation

• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test (662 passed)
• pnpm test:coverage (72.89% statements, 75.50% lines)
• pnpm test:e2e (6 passed)
• pnpm bench:perf (149 sessions; dashboard 1,836 ms; detail 847 ms)

Closes CS-44