chore: housekeep dead artifacts in the state root (owner action)

## What

The state root accumulates dead artifacts: `sessions/` holds 428 files, `model-usage.json` has no writer in current code, and `accounts.json.v1-backup`, `codex-accounts.json.v1-backup`, and stray `*.tmp` files sit beside the live state.

## Why this is wrong

Stale artifacts mislead diagnosis: `model-usage.json` carries a day-old per-model figure that no code reads or writes, and the backups predate the v2 schema.

## Not claimed

No code change proposed. Deletion on the live host is the owner's call, and `sessions/` growth is bounded only by disk.