chore(github): try to have productive copilot reviews

.github/copilot-instructions.md

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+<!--
+  - SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+  - SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+# Copilot Code Review Instructions
+
+## Scope
+Only review for the following:
+- **Bugs**
+- **Security**: Injection vulnerabilities (SQL, command, XSS), hardcoded
+  secrets or credentials, insecure deserialization, broken auth,
+  path traversal, unsafe use of eval or dynamic code execution.
+
+## Strict exclusions — do not comment on these
+- Code style, formatting, or whitespace
+- Naming conventions (variables, functions, classes, files)
+- Suggestions to refactor or restructure working code
+- Performance micro-optimizations unless they cause a measurable performance regression or issue
+- Alternative ways to write functionally equivalent code
+
+## If no bugs or security issues are found
+Leave a short positive review. Example:
+> "No bugs or security issues found. Looks good to me."
+
+## Review format
+Do not include a summary or overview of the changes at the start of the review.
+Go directly to findings, or if there are none, leave only the approval line.
+
+## Comment format (when issues are found)
+For each issue, state:
+1. **Type**: Bug or Security
+2. **Severity**: Critical / High / Medium
+3. **Problem**: What is wrong and why it matters
+4. **Suggestion**: A concrete fix, not a vague recommendation
+
+Do not leave comments that don't fit this format.