feat(acp): wire ACP create_artifact to the document model pipeline

[paulbreuler]

Problem

The ACP create_artifact tool in src/acp/tools.ts returns a delegation message without actually creating any files. Agents calling it via ACP get no artifact back.

Current behavior

async create_artifact(req) {
  return { ok: true, message: 'Artifact creation delegated to AI agent', request: req };
}

Expected behavior

create_artifact calls the same generateArtifactHandler logic used by the generate_artifact MCP tool — creating the file via the IDocument pipeline and returning the path, id, and content.

Acceptance criteria

• create_artifact({ type, title, planId?, description?, personaPath?, primer? }) — writes the artifact file and returns { ok, id, type, title, path, content }
• Fires the artifact:created hook event after writing
• Supports all four artifact types: persona, journey, spec, agent
• Returns a structured error for unknown types or missing required fields (e.g. planId for agent type)
• Tests cover: persona creation, agent creation with primer injection, missing required field error

Files to modify

• src/acp/tools.ts — call generateArtifactHandler or extract shared logic into a service
• src/tools/generate-artifact.ts — consider extracting core logic into src/cli/generate-artifact.ts for reuse
• tests/acp/tools.test.ts — add artifact creation tests with temp workspace

Related

• src/tools/generate-artifact.ts — existing MCP tool implementation
• src/documents/ — Persona, Journey, Spec, AgentTask document model
• src/hooks/runner.ts — hook event firing