Align ACPX built-in agent registry with latest acpx (#55476)

* Add Cursor CLI to ACP allowedAgents

- acpx: add cursor to ACPX_BUILTIN_AGENT_COMMANDS (agent acp)
- docs: add cursor to acp-agents harness list and allowedAgents example

Fixes #28321

Made-with: Cursor

* ACP Cursor: add to acp-router skill, system-prompt, and schema help

- acp-router SKILL: add Cursor to description, intent, agentId mapping,
  harness aliases, and built-in adapter commands (agent acp)
- system-prompt: add cursor to ACP harness example
- schema.help: add cursor to runtime.acp.agent example

Fixes #28321

Made-with: Cursor

* fix(acpx): align built-in agent registry with latest acpx

---------

Co-authored-by: Rob MacDonald <rob@robmacdonald.com>

src/agents/system-prompt.ts

@@ -435,7 +435,7 @@ export function buildAgentSystemPrompt(params: {
     "If a task is more complex or takes longer, spawn a sub-agent. Completion is push-based: it will auto-announce when done.",
     ...(acpHarnessSpawnAllowed
       ? [
-          'For requests like "do this in codex/claude code/gemini", treat it as ACP harness intent and call `sessions_spawn` with `runtime: "acp"`.',
+          'For requests like "do this in codex/claude code/cursor/gemini" or similar ACP harnesses, treat it as ACP harness intent and call `sessions_spawn` with `runtime: "acp"`.',
           'On Discord, default ACP harness requests to thread-bound persistent sessions (`thread: true`, `mode: "session"`) unless the user asks otherwise.',
           "Set `agentId` explicitly unless `acp.defaultAgent` is configured, and do not route ACP harness requests through `subagents`/`agents_list` or local PTY exec flows.",
           'For ACP harness thread spawns, do not call `message` with `action=thread-create`; use `sessions_spawn` (`runtime: "acp"`, `thread: true`) as the single thread creation path.',