feat: update Copilot instructions for Discord.js 15.0.0-dev and enhance validation steps

.github/prompts/error-strict.prompt.md

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+# Prompt: TypeScript Strict Error Resolver
+
+Whenever Copilot generates or modifies code:
+
+1. **Never ignore compiler errors** like `TS2339`, `TS2345`, or similar.
+    - Always explain what the error means in plain language.
+    - Always suggest at least one concrete fix.
+
+2. **For missing properties or methods (e.g. `TS2339`)**:
+    - Check if the method exists in the installed `node_modules`.
+    - If it doesn’t exist, assume the API changed in `discord.js@15-dev`.
+    - Propose alternatives based on actual available methods (`example.ts.txt` must be consulted).
+
+3. **For type mismatches (e.g. `TS2345`)**:
+    - Suggest code changes that handle `null`/`undefined` safely.
+    - Show how to cast or coerce types **without breaking strict typing**.
+
+4. **Validation step**:
+    - Always remind to rerun:
+      ```bash
+      npx tsc --noEmit
+      ```
+      until the project compiles cleanly.
+    - Do not consider the task “done” if compiler errors remain.
+
+5. **Explicitness**:
+    - Always specify: “This code suggestion resolves TS2339 / TS2345.”
+    - Never produce a code snippet that still triggers the same error.