Persist per-issue token usage

[danielmcauley]

Context

Symphony shows live token totals, but completed tickets lose per-issue token observability after the run exits or the process restarts.

TL;DR

Persist per-issue Codex token totals in a JSONL ledger.

Summary

• Add a durable token usage ledger next to Symphony logs.
• Record live and final high-water token observations per issue/session.
• Expose ledger summaries in the observability API.
• Document durable token accounting and API behavior.

Alternatives

• SQLite was considered, but JSONL fits the existing logs-root model.
• Linear comments were avoided because they are a display surface, not a ledger.

Test Plan

• mise exec -- make all
• mise exec -- mix test test/symphony_elixir/token_usage_ledger_test.exs test/symphony_elixir/orchestrator_status_test.exs test/symphony_elixir/extensions_test.exs
• mise exec -- mix specs.check