Add local token stats by model, provider, and agent

[gaoyu06]

Local Settings → Overview usage now breaks down tokens by channel (provider), model, and coding agent, without changing the existing daily heatmap totals.

Scope

• Extend jucode-usage-daily day buckets with models / agents maps (keep v1 in/out/prov parse).
• Record model + provider + agent (jucode / claude / codex / acp:<id>) from engine usage events.
• Overview: dimension switcher, 15-day window totals with bars, per-day chips for the selected dimension.
• This is local session usage only — not JuCode account API usage.

Ship-ready

• pnpm test: 30 files / 476 tests (12 new in usageStats.test.ts).
• pnpm check: clean.
• No new dependencies.

Needs review

• Persist shape + agent key / ACP label sidecar.
• Overview UI for the three dimensions vs. the unchanged heatmap.

Review preconditions

• Confirm old localStorage payloads still load.
• Confirm Settings → Overview is not mixed with Account usage.
• Visual pass of the dimension switcher and bars (desktop).