chore: add model-usage skill

skills/model-usage/references/codexbar-cli.md

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+# CodexBar CLI quick ref (usage + cost)
+
+## Install
+- App: Preferences -> Advanced -> Install CLI
+- Repo: ./bin/install-codexbar-cli.sh
+
+## Commands
+- Usage snapshot (web/cli sources):
+  - codexbar usage --format json --pretty
+  - codexbar --provider all --format json
+- Local cost usage (Codex + Claude only):
+  - codexbar cost --format json --pretty
+  - codexbar cost --provider codex|claude --format json
+
+## Cost JSON fields
+The payload is an array (one per provider).
+- provider, source, updatedAt
+- sessionTokens, sessionCostUSD
+- last30DaysTokens, last30DaysCostUSD
+- daily[]: date, inputTokens, outputTokens, cacheReadTokens, cacheCreationTokens, totalTokens, totalCost, modelsUsed, modelBreakdowns[]
+- modelBreakdowns[]: modelName, cost
+- totals: totalInputTokens, totalOutputTokens, cacheReadTokens, cacheCreationTokens, totalTokens, totalCost
+
+## Notes
+- Cost usage is local-only. It reads JSONL logs under:
+  - Codex: ~/.codex/sessions/**/*.jsonl
+  - Claude: ~/.config/claude/projects/**/*.jsonl or ~/.claude/projects/**/*.jsonl
+- If web usage is required (non-local), use codexbar usage (not cost).