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/scripts/sheet-metal.json

@@ -0,0 +1,9 @@
+{"commands": [
+  {"command": "sheet.manage_rules", "params": {"name": "Steel 1.5", "thickness": "1.5 mm", "bend_radius": "1.5 mm", "k_factor": 0.44, "active": true}},
+  {"command": "sketch.create", "params": {"plane": "XY", "name": "Base"}},
+  {"command": "sketch.rectangle.two_point", "params": {"p0": [0, 0], "p1": [80, 50]}},
+  {"command": "sketch.finish", "params": {}},
+  {"command": "sheet.flange", "params": {"type": "base"}},
+  {"command": "sheet.flange", "params": {"edges": [[40, 0, 1.5]], "height": 20}},
+  {"command": "sheet.flange", "params": {"edges": [[40, 50, 1.5]], "height": 20}}
+]}