docs(issue2): add Hy3 integration guides and Evidence Board demo

[dujunjin]

Related to #2

What changed

• add bilingual end-user integration guides for OpenRouter, Aider, Cline, Continue, and Roo Code
• add a bilingual integration index with shared endpoint presets, acceptance commands, and security guidance
• add Hy3 Evidence Board, a dependency-free web showcase that uses an OpenAI-compatible Hy3 endpoint and a bounded search_knowledge_base tool loop
• add a deterministic offline mode for local validation without impersonating live Hy3 output
• add desktop/mobile screenshots and an 8-second offline walkthrough GIF
• ignore local .env variants while retaining .env.example

Why

Issue #2 asks for at least five complete product integration guides plus a small Hy3-powered showcase. This change provides a reproducible implementation with clear separation between offline verification and live-model evidence.

User and developer impact

• users can configure either OpenRouter (tencent/hy3) or a self-hosted OpenAI-compatible endpoint (hy3)
• API keys remain server-side and are never persisted by the browser
• the demo has no third-party runtime dependencies and can be extracted into a standalone repository
• tool access is limited to read-only bundled knowledge-base search, with request-size, question-length, and tool-round bounds

Validation

• python3 -m unittest discover -s tests -v — 13 tests passed
• python3 server.py --check — deterministic retrieval/tool-loop self-check passed
• python3 -m py_compile evidence_board.py server.py
• node --check static/app.js
• Playwright desktop and 390px mobile flows — 0 console errors and 0 warnings
• Markdown local-link check, git diff --check, and secret-pattern scan passed

Evidence boundary

The committed screenshots and GIF are visibly labeled offline demonstrations. They validate the app, retrieval path, and responsive UI but do not claim that a live Hy3 request was made. A live endpoint and API key are intentionally not included in the repository.