feat: clean-room synthesis — prompt-engineering skill

Distilled from NeoLab customaize-agent:prompt-engineering rubric (GPL-3.0
source; clean-room, no prose reused). ~17 KB NeoLab version trimmed to
tighter ~430 lines focused on what's genuinely non-obvious:

- Four prompt-type classification (discipline-enforcing / guidance /
  collaborative / reference), with explanations for each so the user
  knows what they're picking. Used in both design and critique modes.
- Seven persuasion principles (Meincke et al. 2025, N≈28,000), with
  by-type matrix. Notably flags Liking as actively harmful for
  collaborative prompts (breeds sycophancy in reviews/critiques).
- Degrees-of-freedom axis (high/medium/low) matched to task fragility.
- Context-window-as-shared-resource framing.
- Brief reference only for classical techniques (few-shot, CoT, system
  prompts, templates) — widely documented elsewhere, not re-taught.
- Explicit ethics test for persuasion use.
- Design-mode vs critique-mode workflows.
- Anti-patterns list covering sycophancy-by-default, hedging-on-
  discipline-prompts, authority-stack-on-guidance, high-freedom-on-
  fragile-tasks.

Landscape: no prompt-engineering skill exists in Anthropic's official
repo, wshobson/agents, or the major community skill collections. Real gap.

Makefile SKILLS extended; global symlink installed.

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