Add Oh My Pi (omp) with built-in ACP

[Gyarados4157]

Adds Oh My Pi (omp) to the ACP registry so Zed and other ACP clients can install it from the registry instead of a hand-written custom agent.

This is not a new adapter. Oh My Pi already ships a built-in ACP server:

omp acp

which is equivalent to:

npx -y @oh-my-pi/pi-coding-agent@18.1.9 acp

Why this PR

• Registry currently has pi-acp (upstream Pi via a third-party adapter). It does not list Oh My Pi.
• Open PRs #301 (omp-acp, 15.9.0) and #502 (omp, 17.3.3) are stale. This one pins the current npm release 18.1.9 and uses the upstream built-in ACP command.

Distribution

• id: omp
• name: Oh My Pi
• npx: @oh-my-pi/pi-coding-agent@18.1.9 with args: ["acp"]
• source: https://github.com/can1357/oh-my-pi
• docs: https://omp.sh/docs/acp
• license: MIT

Once merged, hourly version updates can pick up later npm releases.

Authentication

initialize advertises Agent Auth:

{
  "id": "agent",
  "name": "Use existing local credentials",
  "description": "Authenticate via the provider keys/OAuth state already configured under ~/.omp."
}

Verified locally against the installed omp/18.1.9 binary:

initialize -> protocolVersion 1
agentInfo: { "name": "oh-my-pi", "title": "Oh My Pi", "version": "18.1.9" }
authMethods: [{ "id": "agent", "name": "Use existing local credentials", ... }]

This matches AUTHENTICATION.md (Agent Auth, type defaults to agent).

Validation

SKIP_URL_VALIDATION=1 uv run --with jsonschema .github/workflows/build_registry.py --dry-run

passed and included Added agent: omp v18.1.9. Icon is a 16x16 monochrome currentColor π mark (upstream favicon uses a gradient, which the registry rejects).

After merge, Zed users can install Oh My Pi from zed: acp registry and start an external-agent thread without editing settings.json.