feat(hint): H2 Phase B — per-form keying + the three exemplars (ADR-0053)

The first exemplar (`add 1:n relationship`) showed per-node keying is
too coarse for multi-form commands, so revise the mechanism to per-form.

- CommandNode `hint_id: Option<&str>` -> `hint_ids: &[&str]` (mirrors
  usage_ids); hint_key_for_input_in_mode reuses a factored-out
  pick_form_key (shared digit/m:n/suffix form disambiguation with
  usage_key_for_input_in_mode)
- wire INSERT + ADD (all four forms) with hint_ids
- author the three approved exemplars: hint.cmd.insert,
  hint.cmd.add_relationship, hint.err.foreign_key.child_side
  (what/example/concept) + keys.rs registration
- revise ADR-0053 D3 to per-form; record clause-concept hints as a
  deferred extension (issue #37); update README + plan
- +5 tests; 2488 pass / 1 ignored, clippy clean

src/dsl/walker/mod.rs

@@ -6910,7 +6910,7 @@ mod dispatch_3a_tests {
         shape: Node::Word(Word::keyword("dsltail")),
         ast_builder: dsl_builder,
         help_id: None,
-        hint_id: None,
+        hint_ids: &[],
         usage_ids: &[],
     };
     static SMOKE_SQL: CommandNode = CommandNode {
@@ -6918,7 +6918,7 @@ mod dispatch_3a_tests {
         shape: Node::Word(Word::keyword("sqltail")),
         ast_builder: sql_builder,
         help_id: None,
-        hint_id: None,
+        hint_ids: &[],
         usage_ids: &[],
     };