fix: replace unavailable nano model and add concrete error content in failing samples

[Copilot]

Two rig samples failed in the daily sampler run — one due to an unavailable model, one due to the model receiving no input content and returning plain text instead of JSON.

Changes

• 61-genaiscript-list-files-port.md — change model: "nano" → model: "mini". The nano model is not enabled in the Copilot policy, causing a hard runtime error.

• 25-migration-guide.md — embed a concrete example error in the instructions. The previous instructions gave the model nothing to rewrite, so it returned plain text and failed JSON parsing:

-    instructions: `Rewrite the error to be actionable and precise.`,
+    instructions: `Rewrite the following error to be actionable and precise:\n\nTypeError: Cannot read properties of undefined (reading 'map') at Array.forEach (<anonymous>)\n    at processItems (src/processor.ts:23:18)`,

• Fixes [rig-sampler] Daily Rig sample report — 2026-07-28 #254