docs: split Query from Job, and fold the agent tutorial into Agentic Skills

Query and Job were one page because they share the _async convention, but they
are different things: a query returns rows and streams them, a job returns an
outcome you wait for. Two pages now, query first, each carrying a short pointer
to the other rather than the shared framing.

Agentic Skills absorbs the agent plugin tutorial, which was a separate page in
Use Cases describing the same plugin in more detail -- the two had diverged, with
the tutorial carrying the fuller install instructions. It also moves to Getting
Started: installing the plugin is something you do before writing anything, not
a concept to understand.

docs/web/sql/show.mdx

@@ -49,7 +49,7 @@ SHOW JOBS
 ```
 
 Every job the server knows about — id, type, state, creation time. The SQL
-counterpart of `list_jobs`; see [Query and job](/basics/jobs).
+counterpart of `list_jobs`; see [Jobs](/basics/jobs).
 
 ## SHOW ERRORS {#errors}