feat: add caveman skill with detailed README and SKILL documentation

.agents/skills/caveman/README.md

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+# caveman
+
+Talk like smart caveman. Same brain, fewer tokens.
+
+## What it does
+
+Compress every model response to caveman-style prose. Drops articles, filler, pleasantries, and hedging. Keeps every technical detail, code block, error string, and symbol exact. Cuts ~65-75% of output tokens with full accuracy preserved. Mode persists for the whole session until changed or stopped.
+
+Six intensity levels:
+
+| Level | What change |
+|-------|-------------|
+| `lite` | Drop filler/hedging. Sentences stay full. Professional but tight. |
+| `full` | Default. Drop articles, fragments OK, short synonyms. |
+| `ultra` | Bare fragments. Abbreviations (DB, auth, fn). Arrows for causality. |
+| `wenyan-lite` | Classical Chinese register, light compression. |
+| `wenyan-full` | Maximum 文言文. 80-90% character reduction. |
+| `wenyan-ultra` | Extreme classical compression. |
+
+Auto-clarity rule: caveman drops to normal prose for security warnings, irreversible-action confirmations, multi-step sequences where fragment ambiguity risks misread, and when user repeats a question. Resumes after the clear part.
+
+## How to invoke
+
+```
+/caveman              # full mode (default)
+/caveman lite         # lighter compression
+/caveman ultra        # extreme compression
+/caveman wenyan       # classical Chinese
+stop caveman          # back to normal prose
+```
+
+## Example output
+
+Question: "Why does my React component re-render?"
+
+Normal prose:
+> Your component re-renders because you create a new object reference each render. Wrapping it in `useMemo` will fix the issue.
+
+Caveman (full):
+> New object ref each render. Inline object prop = new ref = re-render. Wrap in `useMemo`.
+
+Caveman (ultra):
+> Inline obj prop → new ref → re-render. `useMemo`.
+
+## See also
+
+- [`SKILL.md`](./SKILL.md) — full LLM-facing instructions
+- [Caveman README](../../README.md) — repo overview, install, benchmarks