feat(hint): clause-concept hints layered on F1 (issue #37)

Add a hint.concept.* tier-3 layer surfaced when the cursor sits inside a
recognized clause (referential actions, 1:n/m:n cardinality, primary key,
unique, check, foreign key), layered beneath the per-form block. New
Node::Concept grammar wrapper records clause byte-spans; concept_topic_at_cursor
resolves the innermost containing span. Examples are mode-keyed so they stay
syntax-correct in both simple and advanced mode. Draft ADR (number at merge).

CHANGELOG.md

@@ -8,6 +8,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Pressing **F1** while the cursor sits inside a clause now adds a short
+  "About this clause" explanation beneath the command hint — covering
+  referential actions (`on delete`/`on update`), one-to-many and
+  many-to-many relationships, primary keys, unique and check constraints,
+  and foreign keys. The example shown matches whichever mode you're in.
 - `help` now covers the advanced-mode SQL commands: `help select`, `help with`,
   and the SQL forms of `insert` / `update` / `delete` / `explain` show their own
   syntax, and the full command list is grouped into "Simple-mode commands" and