feat: evolve Helm chart into full RustDesk stack #9

Implement the approved stack architecture for rustdesk-web-stack.

Scope:

• introduce repository operating context via .context and AGENTS.md, inspired by retex-iconic/rgk but scaled to this project;
• replace the web-only Helm chart with a full rustdesk-stack chart containing web, hbbs and hbbr;
• use the official RustDesk server image pinned independently from the stack version;
• expose browser HTTP/WSS using Kubernetes Gateway API without bundling an ingress/controller;
• keep native RustDesk TCP/UDP exposure configurable through Kubernetes Services;
• replace NGINX in the web runtime with Caddy;
• publish the web image and Helm chart as OCI artifacts under the ghcr.io/nomed/rustdesk-web-stack/... prefix;
• keep one Release Please semantic version for the stack artifacts.

Approved architectural decisions are to be recorded in .context/decisions and implemented through a focused PR.

Acceptance criteria:

• AGENTS.md and .context operating structure exist;
• an accepted ADR records packaging, chart topology, Gateway API and Caddy decisions;
• charts/rustdesk-stack renders web + hbbs + hbbr and Gateway API resources;
• server image/version are separately configurable and pinned by default;
• release workflow publishes ghcr.io/nomed/rustdesk-web-stack/web:<version> and oci://ghcr.io/nomed/rustdesk-web-stack/charts/rustdesk-stack:<version>;
• old charts/rustdesk-web references are removed;
• web runtime no longer uses NGINX;
• Helm lint/template checks are wired into CI/release validation.