Consolidates AI agent instructions into the standard AGENTS.md location (repo root) and adds explicit agent responsibility requirements.

Changes

• Renamed .github/copilot-instructions.md → AGENTS.md (repo root — standard location recognised by AI coding agents)
• Added Agent Responsibilities section near the top with four new requirements:
  • SHOULD run all relevant tests and resolve failures per repo convention
  • SHOULD run all relevant linters and resolve issues per repo convention
  • MUST verify and summarise visual changes when site output is affected
  • MUST smoke test desk user workflows by running commands as a user would

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