docs: add mxbuild setup & mx tool skill

[engalar]

Summary

• Add mxbuild-setup.md skill to .claude/skills/mendix/ for end-user delivery via mxcli init
• Covers mxbuild download, mx check validation, and mx create-project usage
• Addresses common AI agent mistakes: incorrect mx create-project syntax, missing mxcli docker check wrapper

Test plan

• Baseline test: agent without skill uses wrong mx create-project syntax (mx create-project /path)
• With skill: agent correctly uses --app-name flag and cwd-based output
• With skill: agent recommends ./mxcli docker check over direct mx check
• With skill: agent uses end-user paths (./mxcli) not dev paths

🤖 Generated with Claude Code

[ako]

Review: Overlap with docker-workflow.md + factual concern

Overlap

Most of this content already exists in .claude/skills/mendix/docker-workflow.md:

• mxcli setup mxbuild with --version, -p, --dry-run variants (lines 78-88)
• mx create-project usage (lines 55-66)
• mxbuild cache locations (line 92)
• Troubleshooting: mxbuild not found, StudioPro.conf.hbs (lines 394-399)
• Runtime-to-mxbuild copying workaround (lines 96-105)

Factual concern

This skill says mx create-project "does NOT accept an output path argument", but docker-workflow.md already documents:

~/.mxcli/mxbuild/{version}/modeler/mx create-project --app-name MyApp --output-dir /path/to/my-app

One of these is wrong. Please verify whether --output-dir exists before merging.

What's worth keeping

• The "Common Workflow: Modify and Validate" 3-step recipe
• Emphasizing ./mxcli docker check over direct mx check for end-user context
• Blank project security caveat (no demo users)

Suggestion

Either trim this skill to only the non-overlapping content, or fold the useful bits into docker-workflow.md instead of creating a second skill that covers the same ground.