[FEATURE]: /usage — dedicated session token & cost report (alias /cost)

Requested feature

Add a /usage slash command (alias /cost) producing a dedicated report of session token consumption and financial cost.

Why

Today cost/token info is only a compact inline line in the sidebar footer (packages/tui/src/feature-plugins/sidebar/context.tsx). A dedicated report would give per-message and cumulative accounting.

Proposed behavior

• /usage opens a report: per-message tokens (input/output/reasoning/cache), cumulative totals, and estimated cost { amount, currency } (data already tracked, see packages/opencode/src/acp/usage.ts).
• Showables: per session, per day, or since a date; exportable as markdown.
• Optionally aggregate across all providers used.

Benefits

• Financial/cost visibility (LLM spend) without external tooling.
• Analogous to /cost in other CLIs; complements /context for window size vs /usage for spend.