refactor(cli): convert stats command to effectCmd

[kitlangton]

Summary

• Convert `cli/cmd/stats.ts` from `cmd()` + `bootstrap()` to `effectCmd` + `Effect.ensuring(store.dispose(ctx))`.
• `aggregateSessionStats` previously read `Instance.project` from ALS via a `getCurrentProject()` helper (deleted). Now takes `currentProject` as an explicit third parameter — the handler passes `ctx.project`.

Behavioral notes

• Subagent purity audit: PURE for the only caller.
• `aggregateSessionStats` is exported but has no other callers in the repo. The new throw `"currentProject required when projectFilter is empty string"` is unreachable in practice (StatsCommand always passes ctx.project). Defensive scaffolding only.
• Disposal coverage strictly improves (Effect.ensuring covers interruption; legacy try/finally didn't).

Test plan

• `bun run typecheck`
• `bun run test test/cli/` — 137 pass
• `stats --days 30` runs and prints a valid table