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/guide/shortcuts.md

@@ -0,0 +1,29 @@
+# Keyboard shortcuts
+
+Command shortcuts can be changed in File › Keyboard Shortcuts; these are the defaults.
+
+| Key | Command | Key | Command |
+|---|---|---|---|
+| L | Line | E | Extrude |
+| R | Two-Point Rectangle | Q | Press Pull |
+| C | Center Diameter Circle | F | Fillet |
+| D | Sketch Dimension | H | Hole |
+| X | Construction | M | Move/Copy |
+| O | Offset | J | Joint |
+| T | Trim | I | Measure |
+| P | Project | A | Appearance |
+| Ctrl+Z | Undo | Ctrl+Y | Redo |
+| Ctrl+N | New Design | Ctrl+O | Open |
+| Ctrl+S | Save | Delete | Delete the selection |
+
+Keys handled by the window itself:
+
+| Key | Action |
+|---|---|
+| S | the S box: search every command at the cursor |
+| V | show or hide the selected bodies, sketches and planes |
+| F2 | rename the selected browser or timeline item |
+| F6 | fit the view to the model |
+| Esc | cancel the running command, or clear the selection |
+
+Help › Keyboard Shortcuts in the app lists the current bindings, including any you changed.