Add run-scoped skill and prompt asset management

backend/skills/builtin/valuation_review/SKILL.md

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+---
+name: valuation_review
+description: Estimate fair value and margin of safety using multiple valuation lenses.
+---
+
+# Valuation Review
+
+Use this skill when the task requires determining whether a stock is cheap, expensive, or fairly priced.
+
+## Workflow
+
+1. Use more than one valuation method when possible.
+2. Compare intrinsic value estimates with current market pricing.
+3. Explain the key assumptions behind the valuation view.
+4. State the margin of safety and what could compress or expand it.
+
+## Guardrails
+
+- Treat valuation as a range, not a single precise number.
+- Call out assumption sensitivity.
+- Avoid high-confidence calls when inputs are sparse or unstable.