Add omp agent to the registry

[lurendie]

Adds omp (Oh My Pi) — a terminal coding agent with a native Rust engine (subagents, plan mode, LSP, DAP, hindsight memory) — to the ACP registry.

• Agent: omp acp — ACP server over stdio (protocolVersion 1)
• Distribution: npm @oh-my-pi/pi-coding-agent via npx, bin omp + args ["acp"] — versions auto-update from npm
• Authentication: verified — initialize returns authMethods: [{"id":"agent","name":"Use existing local credentials",...}] (Agent Auth, type defaults to agent per AUTHENTICATION.md)
• Source: https://github.com/can1357/oh-my-pi (MIT)
• Icon: custom 16x16 monochrome currentColor Pi symbol (upstream asset is 120x90 with hardcoded colors, so it cannot be used as-is)

Handshake verified against omp 17.2.4/17.3.3 in an isolated HOME (sandbox): session capabilities + agent auth method advertised correctly.