docs: add mxbuild setup & mx tool skill

.claude/skills/mendix/mxbuild-setup.md

@@ -0,0 +1,90 @@
+# mxbuild Setup & mx Tool Skill
+
+This skill covers downloading mxbuild and using the `mx` tool for project validation and creation.
+
+## When to Use This Skill
+
+Use this when:
+- The user needs to validate a project with `mx check`
+- The user wants to create a blank Mendix project from scratch
+- The user encounters "mx not found" or mxbuild-related errors
+- You need to run `mx check` after modifying an MPR file via MDL
+
+## Setup: Download mxbuild
+
+mxbuild is downloaded once and cached at `~/.mxcli/mxbuild/{version}/`.
+
+```bash
+# Auto-detect version from project (recommended)
+./mxcli setup mxbuild -p app.mpr
+
+# Or specify version explicitly
+./mxcli setup mxbuild --version 11.6.3
+```
+
+After download, the `mx` binary is at: `~/.mxcli/mxbuild/{version}/modeler/mx`
+
+## Validate a Project
+
+**Preferred** — use the mxcli wrapper (auto-resolves mx binary):
+
+```bash
+./mxcli docker check -p app.mpr
+```
+
+**Direct** — call mx binary explicitly:
+
+```bash
+~/.mxcli/mxbuild/11.6.3/modeler/mx check /path/to/app.mpr
+```
+
+Reports errors (CE-codes), warnings, and deprecations. Non-zero exit on errors.
+
+## Create a Blank Project
+
+**IMPORTANT:** `mx create-project` uses `--app-name` and outputs `App.mpr` in the **current directory**. It does NOT accept an output path argument.
+
+```bash
+# 1. Ensure mxbuild is available
+./mxcli setup mxbuild --version 11.6.3
+
+# 2. Create project directory and cd into it
+mkdir /tmp/my-project && cd /tmp/my-project
+
+# 3. Create blank project (outputs App.mpr in current directory)
+~/.mxcli/mxbuild/11.6.3/modeler/mx create-project --app-name MyApp
+```
+
+Takes ~29 seconds. Creates an MPR v2 project with standard module structure.
+
+**Caveat:** Blank projects have no demo users — login will fail until you configure security via MDL or Studio Pro. See `manage-security.md` for setting up demo users.
+
+## Common Workflow: Modify and Validate
+
+```bash
+# 1. Download mxbuild (once)
+./mxcli setup mxbuild -p app.mpr
+
+# 2. Make changes via MDL
+./mxcli exec changes.mdl -p app.mpr
+
+# 3. Validate
+./mxcli docker check -p app.mpr
+```
+
+## Troubleshooting
+
+| Problem | Solution |
+|---------|----------|
+| `mx: not found` | Run `./mxcli setup mxbuild -p app.mpr` |
+| Wrong Mendix version | Use `--version` flag or check project version with `./mxcli -p app.mpr -c "SHOW VERSION"` |
+| arm64 download fails | Some older versions lack arm64 builds; use Docker or Rosetta |
+| `mx check` CE errors | See `cheatsheet-errors.md` for common error codes |
+| `mx create-project` hangs | Ensure sufficient disk space; ~29s is normal |
+
+## Related Skills
+
+- [docker-workflow.md](./docker-workflow.md) — Full Docker build and run workflow
+- [run-app.md](./run-app.md) — Running the app after validation
+- [check-syntax.md](./check-syntax.md) — MDL syntax validation before `mx check`
+- [manage-security.md](./manage-security.md) — Setting up demo users for blank projects