Documentation site and CI workflows

An mdBook under docs/book: a user guide (getting started, UI tour, sketching, features,
parameters, components and joints, sheet metal, plastic, files, shortcuts, MCP) and a developer
guide (CONTRIBUTING, architecture, command engine, kernel boundary, persistent naming, test
harnesses, and the existing docs/*.md pages). `cargo xtask book` generates the command reference
from `solvecraft-cli commands` and renders the screenshots with `solvecraft-cli snapshot`, then
runs mdbook. .github/workflows/pages.yml publishes the book at /solvecraft/ and the web app at
/solvecraft/app/ (skipped while the repository is private). .github/workflows/ci.yml runs
`cargo xtask ci` on pull requests. README and Help → Documentation link to the book.

Fixes #28
Fixes #8

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/introduction.md

@@ -0,0 +1,32 @@
+# SolveCraft
+
+SolveCraft is an open-source parametric 3D CAD application written in pure Rust. It works the way
+Fusion-style modellers do: sketch on a plane, constrain and dimension the sketch, turn profiles
+into solids with features on a timeline, and change a parameter to watch the whole timeline
+rebuild.
+
+![The sample plate in SolveCraft](generated/sample-plate.png)
+
+It runs as:
+
+- a **desktop app** for Windows and Linux (egui + wgpu);
+- a **web app** in the browser (wasm, WebGPU or WebGL2), [try it here](app/);
+- a **headless CLI**, `solvecraft-cli`, that runs command scripts, measures and exports;
+- an **MCP server** that AI agents model with, headless or driving the running app.
+
+Everything the user can do is a *command* with JSON parameters. The toolbar, the dialogs, scripts,
+the CLI, the control channel and MCP all run the same commands, so anything you can click you can
+also script, test or hand to an agent.
+
+SolveCraft is early. Sketching, solid modelling, parameters, components with joints, sheet metal
+and a plastic-enclosure subset work; surfaces, drawings, CAM and simulation are deferred. The
+[roadmap](https://github.com/bherbruck/solvecraft/blob/main/ROADMAP.md) has the details, and
+[command parity](dev/parity.md) and the [Fusion oracle](dev/oracle.md) measure how far along it
+is.
+
+This book has two halves: the **user guide** for people modelling with SolveCraft, and the
+**developer guide** for people working on it. The [command reference](reference/commands.md) is
+generated from the code on every build.
+
+SolveCraft is licensed MIT OR Apache-2.0. It is a clean-room implementation and is not affiliated
+with Autodesk; Fusion is a trademark of Autodesk, Inc.