docs(tufte-press): add guardrails and self-check rubric to agent instructions

- Add guardrails for under-specified requests (scaffold with clarifications)
- Add self-check rubric for content quality validation
- Ensures agent validates accuracy, clarity, annotation utility, balance, and print suitability
- Matches tufte-press project's latest LLM authoring prompt

These sections guide the agent to produce high-quality educational content
and handle unclear requests gracefully by asking for clarification.

skills/tufte-press/SKILL.md

@@ -176,6 +176,20 @@ Ask clarifying questions if needed:
 - Lists use JSON arrays not strings
 - Sources are real (or marked with "[NEEDS CLARIFICATION]")
 
+**Guardrails:**
+- If the request is under-specified, return a minimal 1-page scaffold and store 3–5 numbered clarification questions inside `metadata.provenance.notes`
+- Otherwise produce the full card
+- Validate internally before saving
+
+**Self-Check Rubric:**
+Before finalizing, adjust content to meet:
+- **Accuracy**: No speculation, cite real sources
+- **Clarity**: Short sentences, clear definitions
+- **Annotation utility**: Margin notes actionable and self-contained
+- **Balance**: Main content vs margin notes flow naturally
+- **Schema validity**: All required fields present
+- **Print suitability**: Short margin notes (15-25 words), avoid long lines
+
 ### Step 3: Save JSON to File
 
 Write the generated JSON to a file in an appropriate location: