feat: simple-mode code-block highlighting, prompt, and copy rules

Add a custom Shiki grammar for the simple-mode command language
(src/grammars/rdbms.mjs), registered with Expressive Code. Two language ids
share it: rdbms (real commands) and rdbms-syntax (abstract templates).
Simple-mode blocks now highlight; advanced examples keep sql.

Separation + copy ergonomics via CSS (global.css): a decorative, copy-safe
"> " prompt on rdbms command lines (not in the copy buffer), and the copy
button hidden on multi-command rdbms blocks and on rdbms-syntax templates
(the app input is single-line, so a multi-command paste is not runnable);
single-command, sql, and sh blocks keep copy.

Content: convert 22 simple-mode fences to rdbms; lead the simplest examples
(first project, Tables reference) with bare "with pk" (the beginner default
that creates a ready-made id key), pointing to the named form. Record the
fence + prompt conventions in STYLE.md.

website/STYLE.md

@@ -80,8 +80,25 @@ This is planned and not yet available.
 - Where both modes apply, show the **simple-mode** form and its
   **advanced-mode (SQL)** equivalent — the in-app teaching echo already
   pairs these, so docs mirror it.
+- Prefer **worked examples** (a real command on the library schema) over
+  abstract prose, and **always cross-link** the related reference/guide
+  pages (use stubs so links resolve before a page is written).
 - Code blocks for exact input/output; reserve casts for motion/flow.
 
+### Code-block fences
+
+- **Simple-mode commands → ` ```rdbms `** — custom highlight grammar in
+  `src/grammars/rdbms.mjs`, registered with Expressive Code in
+  `astro.config.mjs` (keywords + types coloured).
+- **Advanced-mode SQL → ` ```sql `**; shell / install → ` ```sh `.
+- A decorative `> ` prompt is prepended to `rdbms` lines via CSS
+  (`src/styles/global.css`) — **do not type `>` in the fence**. It is
+  copy-safe (Expressive Code's copy button uses `data-code`) and
+  `user-select:none`.
+- **One command per line** in an `rdbms` block; a multi-line single
+  statement (e.g. advanced `CREATE TABLE`) belongs in a ` ```sql ` block,
+  where no prompt is added.
+
 ### Canonical library schema (source of truth for examples)
 
 Use these exact names/types in every example: