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/strings/en-US.yaml

@@ -391,6 +391,27 @@ hint:
   # H2 / ADR-0053: shown by `hint` / F1 when there is nothing specific
   # to expand on (no recent error, empty input).
   getting_started: "Start typing a command and press F1 for a hint, or type `help` for the full command list."
+  # ── Tier-3 teaching blocks (ADR-0053 D3) ──────────────────────────
+  # Per-form command hints (`hint.cmd.<form>`) and per-class error
+  # hints (`hint.err.<class>`), each a `what` (1–2 sentences) / `example`
+  # (one runnable, mode-correct line) / `concept` (the relational idea —
+  # the teaching part). Phase B seeds the three approved exemplars; the
+  # rest are authored in Phase C.
+  cmd:
+    insert:
+      what: "Add one or more rows to a table."
+      example: "insert into Customers values ('Ann', 'ann@example.io')"
+      concept: "A row is one record; each value lines up with a column, in order. Columns typed `serial`/`shortid` fill themselves — leave them out."
+    add_relationship:
+      what: "Link two tables so a parent row can own many child rows."
+      example: "add 1:n relationship from Customers.id to Orders.customer_id"
+      concept: "The \"1:n\" means one parent, many children. The child column holds the foreign key; add `--create-fk` to create that column if it doesn't exist yet."
+  err:
+    foreign_key:
+      child_side:
+        what: "The value you gave for the child column doesn't match any parent row, so the foreign key has nothing to point at."
+        example: "First insert the parent (insert into Customers …), then the child that references it."
+        concept: "A foreign key is a promise that every child points at a real parent, so the parent must exist first. To allow orphans on delete instead, set the relationship's `on delete` to `set null` or `cascade`."
   # Invalid identifier in a schema slot (ADR-0022 stage 8e
   # + the user's #5). Voice mirrors ADR-0019's "no such
   # {kind}" wording for consistency with engine errors.