Add kilocode as a built-in agent

[kimptoc]

Kilocode supports ACP natively via kilo acp / kilocode acp and is published as @kilocode/cli on npm, enabling zero-setup usage consistent with other built-in agents.

Note: kilocode can already be used today via the escape hatch: acpx --agent "kilo acp" "your prompt" — this issue tracks adding it as a first-class built-in for zero-config use.

Proposed change to src/agent-registry.ts

```typescript
export const AGENT_REGISTRY: Record<string, string> = {
codex: "npx @zed-industries/codex-acp",
claude: "npx -y @zed-industries/claude-agent-acp",
gemini: "gemini",
opencode: "npx -y opencode-ai acp",
pi: "npx pi-acp",
kilocode: "npx -y @kilocode/cli acp",
};
```

Users can then run:
```bash
acpx kilocode "help me debug this code"
acpx kilocode sessions new --name my-project
acpx kilocode --session my-project "continue working"
```

@kilocode/cli is auto-downloaded on first use via npx — no manual installation required.

Checklist

• Add kilocode: "npx -y @kilocode/cli acp" to AGENT_REGISTRY
• Update test/agent-registry.test.ts count from 5→6 and include kilocode in the set assertion