feat: support explain over advanced-mode SQL queries

explain now wraps the advanced SQL commands — select, with (CTE),
insert, update, delete — in addition to the DSL show data/update/
delete it already covered, rendering through the same plan tree
(ADR-0039, closing the ADR-0030 OOS-2 gap).

Implemented as a second Advanced `explain` CommandNode under the
shared entry word, reusing the established shared-word dispatch
(SQL-first, DSL-fallback) rather than new grammar machinery.
build_explain_sql slices the inner SQL off the source and reuses the
existing SQL builders; do_explain_plan runs EXPLAIN QUERY PLAN over
the carried text verbatim (never executes, so safe for destructive
verbs). Advanced explain update/delete now route through SQL with an
identical plan; DSL-explain tests pinned to simple mode. Help and
usage text now list the advanced explain forms.

CLAUDE.md

@@ -101,8 +101,10 @@ Current decisions at a glance (each backed by an ADR):
 - **Indexes & query plans:** `add index` / `drop index`
   (ADR-0025); `explain show data|update|delete` runs
   `EXPLAIN QUERY PLAN` and renders an annotated, span-styled
-  plan tree (ADR-0028). `EXPLAIN QUERY PLAN` never executes,
-  so explaining a destructive command is safe.
+  plan tree (ADR-0028). In advanced mode `explain` also wraps
+  SQL `select` / `with` / `insert` / `update` / `delete`
+  (ADR-0039). `EXPLAIN QUERY PLAN` never executes, so
+  explaining a destructive command is safe.
 
 ## Repository layout