VS Code: Persist and display token usage by turn/session/history

[benjschiller]

Summary

Token usage is currently visible in some runtime views but not consistently persisted or shown across chat/session/history contexts. We want token usage to be reliable, local-first, and user-configurable in one place.

Problem

• Token counts are not consistently persisted at the turn level in session artifacts.

Proposal

1. Persist token usage per assistant turn in session history artifacts.
2. Compute per-session and history-bucket totals from persisted session data.
3. Add a single setting:
   • continue.showTokenUsage: "never" | "history" | "session" | "turn"
4. Setting semantics are escalating:
   • never: show nowhere
   • history: history only
   • session: history + session
   • turn: history + session + turn
5. Apply setting changes live without requiring Developer: Reload Window.
6. Keep existing stats DB/schema unchanged (no migration in this change).
7. Keep token tracking always-on; setting controls display only.

PR incoming :)