docs: add Copilot instructions and v2→v3 migration prompt for plugin developers

[Copilot]

Adds Copilot-ready developer resources targeting plugin authors consuming the Essentials framework — not framework contributors (separate issue).

Changes

• .github/copilot-instructions.md — Plugin-scoped Copilot instructions covering factory/device/properties class structure, EssentialsDevice vs EssentialsBridgeableDevice, feedback pattern, CommFactory injection, SIMPL bridging, JSON config conventions, and naming rules. Links to hosted docs.

• .github/prompts/migrate-plugin-v2-to-v3.prompt.md — Reusable Copilot prompt that drives migration from the deprecated v2 pattern (static LoadPlugin() / IBridge) to v3 (EssentialsPluginDeviceFactory<T> / EssentialsBridgeableDevice). Includes side-by-side before/after snippets for factory creation, device base class, bridge signature, and properties extraction, plus a completion checklist.

v2 → v3 at a glance

	v2 (deprecated)	v3 (current)
Entry point	static void LoadPlugin() on device class	EssentialsPluginDeviceFactory<T> subclass
Type registration	DeviceFactory.AddFactoryForType(name, fn)	TypeNames list on factory
SIMPL bridge	IBridge + LinkToApi(BasicTriList, uint, string)	EssentialsBridgeableDevice + LinkToApi(…, EiscApiAdvanced)
Min version	static string on device class	Property on factory class

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.