chore: add model-usage skill

skills/model-usage/SKILL.md

@@ -0,0 +1,45 @@
+---
+name: model-usage
+description: Use CodexBar CLI local cost usage to summarize per-model usage for Codex or Claude, including the current (most recent) model or a full model breakdown. Trigger when asked for model-level usage/cost data from codexbar, or when you need a scriptable per-model summary from codexbar cost JSON.
+metadata: {"clawdbot":{"emoji":"📊","os":["darwin"],"requires":{"bins":["codexbar"]},"install":[{"id":"brew-cask","kind":"brew","cask":"steipete/tap/codexbar","bins":["codexbar"],"label":"Install CodexBar (brew cask)"}]}}
+---
+
+# Model usage
+
+## Overview
+Get per-model usage cost from CodexBar's local cost logs. Supports "current model" (most recent daily entry) or "all models" summaries for Codex or Claude.
+
+TODO: add Linux CLI support guidance once CodexBar CLI install path is documented for Linux.
+
+## Quick start
+1) Fetch cost JSON via CodexBar CLI or pass a JSON file.
+2) Use the bundled script to summarize by model.
+
+```bash
+python {baseDir}/scripts/model_usage.py --provider codex --mode current
+python {baseDir}/scripts/model_usage.py --provider codex --mode all
+python {baseDir}/scripts/model_usage.py --provider claude --mode all --format json --pretty
+```
+
+## Current model logic
+- Uses the most recent daily row with `modelBreakdowns`.
+- Picks the model with the highest cost in that row.
+- Falls back to the last entry in `modelsUsed` when breakdowns are missing.
+- Override with `--model <name>` when you need a specific model.
+
+## Inputs
+- Default: runs `codexbar cost --format json --provider <codex|claude>`.
+- File or stdin:
+
+```bash
+codexbar cost --provider codex --format json > /tmp/cost.json
+python {baseDir}/scripts/model_usage.py --input /tmp/cost.json --mode all
+cat /tmp/cost.json | python {baseDir}/scripts/model_usage.py --input - --mode current
+```
+
+## Output
+- Text (default) or JSON (`--format json --pretty`).
+- Values are cost-only per model; tokens are not split by model in CodexBar output.
+
+## References
+- Read `references/codexbar-cli.md` for CLI flags and cost JSON fields.