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/dev/commands.md

@@ -0,0 +1,38 @@
+# The command engine
+
+Every user-visible action is a **command**: a `CommandSpec` registered in
+`crates/engine/src/cmd/`, with a dotted, lower-case id (`solid.extrude`, `sketch.create`,
+`timeline.rollback`). The toolbar, dialogs, marking menu, S box, scripts, the CLI, the control
+channel and MCP all run commands through `Session::execute(id, params)`. Commands take JSON
+parameters and never open dialogs; a dialog is a front-end form that builds the parameters.
+
+```rust
+CommandSpec::new("solid.fillet", "Fillet", fillet)
+    .at("SOLID", "MODIFY")      // toolbar tab and panel ("" = not on the toolbar)
+    .icon("fillet")             // drawn in code, crates/ui-egui/src/icons.rs
+    .key("F")                   // default shortcut
+    .params("edges: [[x,y,z] | {body, index}]; radius: expr; …"),
+```
+
+`run` is `fn(&mut Session, &Value) -> Result<Value>`, and `enabled` is
+`fn(&Session) -> Result<(), String>`; the error string is the disabled-button tooltip. The
+`params` string is the one-line documentation shown by `solvecraft-cli commands`, MCP's
+`list_commands` and the [command reference](../reference/commands.md).
+
+A command that changes the design is undoable by default: the session snapshots the document
+before it runs, and a failing command leaves the document unchanged. Feature commands add a
+feature to the timeline and let the document evaluate it; they validate their inputs first
+(units, ranges, references) and return `BadParams` instead of building a broken feature.
+
+Ids used before the 2026-10 rename still resolve through `crates/engine/src/legacy_ids.rs`, so
+old scripts, recipes, saved shortcuts and toolbar layouts keep working.
+
+## Adding a command
+
+1. Add a `CommandSpec` to the right file in `crates/engine/src/cmd/` and implement `run`.
+2. Write engine tests that measure the result: volume, area, and face/edge/vertex counts
+   against values you can derive by hand.
+3. The hostile-input test (`hostile_params_never_panic` in `crates/engine/src/tests.rs`) runs
+   your command automatically with every documented parameter set to junk. It must return
+   errors, never panic.
+4. For UI work, add a scenario (see [Test harnesses](testing.md)).