Add .github/copilot-instructions.md for LLM guidance

[Copilot]

LLM pre-training data lacks sufficient libhv-specific knowledge, making AI assistants less effective when working with this codebase.

Adds .github/copilot-instructions.md covering:

• Project overview — architecture, C99/C++11 codebase, relationship to libevent/libev/libuv
• Directory structure — base/, event/, evpp/, http/, ssl/, mqtt/, etc.
• Build system — Makefile and CMake with all WITH_* options
• Coding conventions — naming (hv_ prefix for C, PascalCase in hv:: namespace), header guards, export macros, memory management patterns
• Key API patterns — event loop, TcpServer/TcpClient, HTTP server (gin-style routing), requests-style HTTP client, WebSocket server/client
• HTTP handler chain — headerHandler → preprocessor → middleware → processor → postprocessor, handler types, return value semantics (HTTP_STATUS_NEXT/HTTP_STATUS_CLOSE)
• Thread safety — which operations are safe cross-thread (hio_write, hloop_post_event, channel->write)
• Common pitfalls — Windows DELETE macro conflict, HLOOP_FLAG_AUTO_FREE lifetime, async handler return values, IO thread blocking