refactor(storage): remove the unused legacy Usage session scanner

## Context

Desktop Usage now reads canonical data through the selected Runtime Host. The legacy Storage path remains in SettingsStore, but repository-wide production-call-site inspection finds no consumer outside its own definition and dedicated tests.

## Scope

- Remove SettingsStore.usageStats and its readUsageStats import.
- Remove packages/storage/src/usage-stats-store.ts.
- Remove dedicated legacy-reader tests and stale public-entrypoint expectations.
- Verify repository-wide references, package entrypoints, Storage tests, Desktop tests, typecheck, and dependency analysis.

## Non-goals

- Removing SettingsStore itself.
- Removing projectDesktopUsageStats; it still projects Host Session IDs into Desktop namespaces.
- Changing Runtime Host Usage, provenance, pricing, or UI behavior.
- Changing the Runtime Host wire contract or compatibility epoch.

## Acceptance

- No production or test reference remains to the legacy scanner.
- Desktop Usage continues to use the selected Runtime Host.
- Storage and Desktop focused validation remains green.

## Related

- #2128 tracks Usage coverage/presentation.
- #2641 is the superseded broad implementation branch.