feat: deliver full RustDesk stack chart

[nomed]

Closes #9.

Implements the approved full-stack architecture:

• adds RGK-inspired .context and AGENTS.md operating governance;
• records ADR-0001 for packaging/deployment decisions;
• replaces NGINX with a project-owned Caddy runtime;
• replaces the web-only chart with rustdesk-stack containing web, hbbs and hbbr;
• pins official RustDesk Server 1.1.16 independently from the stack version;
• adds optional Kubernetes Gateway API HTTP/WSS routing without bundling a controller;
• moves OCI artifacts under ghcr.io/nomed/rustdesk-web-stack/web and ghcr.io/nomed/rustdesk-web-stack/charts/rustdesk-stack;
• updates Release Please and CI validation accordingly.

CI must validate Helm lint/template (default and Gateway enabled) plus the pinned web build and Caddy repackaging before merge.