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/friendly/keys.rs

@@ -224,6 +224,16 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ),
     ("hint.ambient_expected", &["expected"]),
     ("hint.getting_started", &[]),
+    // Tier-3 teaching blocks (ADR-0053 D3) — Phase-B exemplars.
+    ("hint.cmd.insert.what", &[]),
+    ("hint.cmd.insert.example", &[]),
+    ("hint.cmd.insert.concept", &[]),
+    ("hint.cmd.add_relationship.what", &[]),
+    ("hint.cmd.add_relationship.example", &[]),
+    ("hint.cmd.add_relationship.concept", &[]),
+    ("hint.err.foreign_key.child_side.what", &[]),
+    ("hint.err.foreign_key.child_side.example", &[]),
+    ("hint.err.foreign_key.child_side.concept", &[]),
     (
         "hint.ambient_invalid_ident",
         &["kind", "found"],