improve repo status reporting

[elithrar]

This fixes confusing artifact-fs status output and makes hydration progress visible.

The CLI previously printed Go's zero time for repos that had never fetched, which looked like a real timestamp. It also gave no direct signal for how much of a repo had been hydrated, so ahead and behind could be misread as hydration progress.

• render unset fetch state as never instead of 0001-01-01T00:00:00Z
• include hydrated_blobs and hydrated_bytes in repo status output
• add focused tests for CLI formatting and persisted hydration stats
• clarify the README overview text

repo=workerd state=mounted head=<oid> ref=main ahead=0 behind=0 diverged=false last_fetch=2026-04-01T12:20:52-05:00 result=ok hydrated_blobs=29 hydrated_bytes=448844 overlay_dirty=false

Tests: go test ./internal/cli ./internal/daemon, go vet ./...