Eliminate duplication between copilot-instructions.md and AGENTS.md

[Copilot]

The initial copilot-instructions.md duplicated tech stack, commands, and architecture details already present in AGENTS.md. Revised to establish single source of truth.

Changes

.github/copilot-instructions.md (168→70 lines, -58%)

• Delegates all project details to AGENTS.md via prominent reference at top
• Retains only Copilot-specific workflow guidance: pre-task checklist, code quality standards, common pitfalls
• Emphasizes critical requirement to consult hoist-react documentation before client-side work

AGENTS.md (126→155 lines, +23%)

• Added "Testing & Validation" section documenting manual testing approach
• Added "Security & Boundaries" section:
  • What to modify: app code, configs, docs
  • What NOT to modify: .env, framework code, build configs, certs, Auth0
  • Security guidelines: secrets handling, dependency scanning
• Preserved all existing content (additions only, no replacements)

Result

Clear separation: AGENTS.md contains comprehensive project information for all AI agents; copilot-instructions.md provides Copilot workflow guidance. Tech stack and command updates now require single location maintenance.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #804

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