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/sketching.md

@@ -0,0 +1,34 @@
+# Sketching and constraints
+
+A sketch lies on a plane: an origin plane, a construction plane or a planar face. Start one with
+**Create Sketch** (`sketch.create`) and click a plane, or pick a face first. **Finish Sketch**
+returns to the model.
+
+## Geometry
+
+Lines and polylines (L), rectangles (two-point R, three-point, centre), circles (centre C,
+two-point, three-point, tangent), arcs (three-point, centre, tangent), polygons, slots, ellipses,
+fit-point and control-point splines, conics, points and text. **Construction** (X) turns curves
+into reference geometry that doesn't form profiles. Modify tools include trim (T), extend, break,
+fillet, chamfer, offset (O), mirror, patterns, move/copy and scale.
+
+**Project** (P) and **Intersect** bring model edges into the sketch as linked geometry that
+follows later changes. Sketching on a face projects its edges automatically.
+
+## Constraints and dimensions
+
+The solver is SolveCraft's own (damped Gauss–Newton with degree-of-freedom analysis). Constraints:
+coincident, horizontal/vertical, parallel, perpendicular, tangent, equal, concentric, collinear,
+midpoint, symmetry, fix, curvature (G2) and polygon. **Sketch Dimension** (D) adds linear,
+aligned, horizontal, vertical, radius, diameter and angle dimensions. Each is a parameter
+(`d1`, `d2`, …), so it can hold an expression.
+
+Under-constrained geometry is blue and fully constrained geometry turns dark, and the status
+shows the remaining degrees of freedom. A constraint or dimension that would over-constrain the
+sketch is refused rather than applied. Drag a point to move it; the solver keeps every
+constraint while you drag.
+
+## Profiles
+
+Closed loops become profiles automatically, including regions split by crossing lines and
+islands inside outlines. Extrude, revolve, sweep and loft pick profiles.