ModelCraft foundation: workspace, quality gates, FE model, command engine, geometry boundary

A pure-Rust CAE modeling workspace following the SolidCraft conventions:
- modelcraft-model: nodes and elements by label with hostile-input validation
- modelcraft-geometry: SolidCraft's kernel (pinned git dependency) behind our own API;
  tessellation into a face-tagged boundary surface for volume meshing
- modelcraft-engine: CommandSpec registry, Session::execute with rollback and panic guard,
  scripts and a journal
- modelcraft-cli: list commands, run scripts
- xtask: layers (only geometry may use solidcraft-*), assets, licences, tests, wasm, ci
- AGENTS.md, architecture, ADR 0001 (libraries and licences), roadmap, CI

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Contributing
+
+Thanks for helping. Read [AGENTS.md](AGENTS.md) first: it holds the rules every change follows,
+whether a person or an agent writes it.
+
+1. Branch from `main`, keep a pull request to one topic.
+2. Run `cargo xtask ci` before pushing; CI runs the same gates.
+3. User-visible behaviour is a command (`crates/engine/src/cmd/`) with tests, including hostile
+   parameters.
+4. Meshing and solver algorithms cite the paper they come from in the module docs.
+5. Never contribute code, text or data from GPL-family projects or commercial CAE products
+   (see [docs/adr/0001-libraries.md](docs/adr/0001-libraries.md)).
+
+By contributing you agree your work is licensed under MIT OR Apache-2.0.