[Feature]: OCI containerisation & immutable runtime isolation for boj-server

[hyperpolymath]

Problem / motivation

Launching boj-server via npx assumes prerequisite runtimes (e.g. Deno) on PATH; when missing, the server crashes with unexpected EOF during MCP init.

Proposed solution

• Official OCI image ghcr.io/hyperpolymath/boj-server:latest bundling Deno + Idris2 kernel + core binaries.
• Standardised mcp_config.json using docker run instead of npx for an immutable environment.

Rough scope

Medium. Runtime hardening, not a pipeline stage.

Part of #274. Re-file of #262 (mis-templated).