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/src/content/docs/getting-started/first-project.md

@@ -15,41 +15,41 @@ and press <kbd>Enter</kbd> to run them.
 
 ## Create a table
 
-A table needs at least a primary key. The `with pk` clause names the
-primary-key column and its type:
+The quickest way to make a table is `with pk` on its own, which gives you a
+ready-made primary key column called `id`:
 
-```text
-create table authors with pk author_id(serial)
+```rdbms
+create table authors with pk
 ```
 
-`serial` is an auto-incrementing number — you will not have to fill it in
-yourself.
+You never fill `id` in yourself — the database assigns it as you add rows.
+(You can also name and type the key yourself; see the
+[Tables reference](/reference/tables/).)
 
 ## Add a couple of columns
 
 In simple mode you create a table with its key, then add the other columns
 one at a time:
 
-```text
+```rdbms
 add column to authors: name (text)
 add column to authors: birth_year (int)
 ```
 
 ## Add a row
 
-`insert` adds a row. List the columns you are supplying — the `author_id`
-fills itself in because it is a `serial`:
+`insert` adds a row. List the columns you are supplying — `id` fills itself
+in automatically:
 
-```text
+```rdbms
 insert into authors (name, birth_year) values ('Ada Lovelace', 1815)
 ```
 
-The playground shows the row it just inserted, including the generated
-`author_id`.
+The playground shows the row it just inserted, including the generated `id`.
 
 ## Look at the data
 
-```text
+```rdbms
 show data authors
 ```
 

website/src/content/docs/getting-started/modes.md

@@ -13,7 +13,7 @@ in **simple mode**, and reach for **advanced mode** when you want full SQL.
 Simple mode is a friendly, keyword-based command language designed for
 learning. Commands read close to English:
 
-```text
+```rdbms
 create table authors with pk author_id(serial)
 show data authors
 ```
@@ -33,7 +33,7 @@ select title, published from books where published >= 2000 order by published;
 
 Switch modes with the `mode` command:
 
-```text
+```rdbms
 mode advanced
 mode simple
 ```
@@ -46,7 +46,7 @@ open it, so a project set up for SQL practice reopens in advanced mode.
 When you are in simple mode and want to run a single SQL statement without
 switching, prefix the line with a colon:
 
-```text
+```rdbms
 :select count(*) from books
 ```