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/sheet-metal.md

@@ -0,0 +1,15 @@
+# Sheet metal
+
+The SHEET METAL tab builds parts from a sheet of constant thickness under a **sheet metal rule**
+(thickness, bend radius, K-factor), set in **Sheet Metal Rules** (`sheet.manage_rules`).
+
+- **Flange** (`sheet.flange`): a base flange from a sketch profile, a contour flange from an open
+  sketch, or an edge flange from a part edge.
+- **Hem** (`sheet.hem`) and **Fold** (`sheet.fold`) along a sketch line.
+- **Unfold / Refold** (`sheet.unfold`, `sheet.refold`) for working on the flat part.
+- **Convert** (`sheet.convert`) turns a solid of constant thickness into sheet metal.
+- **Create Flat Pattern** (`sheet.flat_pattern`) shows the flat part with its bend lines and a
+  bend table, and exports it as DXF.
+
+Bends follow the rule's K-factor, so flat lengths match what a press brake will make. The
+oracle checks these against parts built in Fusion.