docker/hsp repository overview

HSP-capable Pi kit -- the `pi` terminal coding agent (with pi-mcp-adapter's hosted MCP gateway self-registration) plus this repo's pi HSP adapter baked into the image, wired up so the session declares Harness Steering Protocol (hsp/1) support. The base image builds directly from the Docker-owned docker/sandbox-templates:shell-docker template (see pi/kit/Dockerfile), reproducing the same pi install steps docker/sbx-kits-contrib's own `pi` kit uses plus pi-mcp-adapter -- no dependency on any externally-hosted derivative image. Built to test/demo the Agentic Platform's custom-kit-only HSP capability (APT-1382).