Add Copilot review instructions

.github/copilot-instructions.md

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+# Copilot code review instructions
+
+Report only correctness, security, data-loss and permission problems. Skip style, naming, comment wording, formatting and test structure. Don't repeat a point that was already declined with a reason in the PR. If a finding wouldn't cause a bug or a leak, don't post it. Prefer one precise comment with the failing input over several general ones.
+
+## Security checklist
+
+Flag any of these:
+
+- Secrets or tokens in logs, error messages or commits.
+- Text written by other people (mail, messages, web pages) reaching a model unwrapped. mcp-hub wraps it in `<untrusted>`.
+- One account's data reaching another account (the agents dashboard is multi-account).
+- Personal data (mail, messages, health) in logs or error messages.
+- A new route or action with no permission check.
+- Workflow changes that give a branch or fork access to secrets (`pull_request_target`, broader `permissions`, secrets passed to unreviewed code).