fix(hint): correct H2 corpus errors + add parse guard (handoff-71)

Semantic verification pass over the tier-3 `hint` corpus (ADR-0053).
Four content errors corrected in src/friendly/strings/en-US.yaml:

- cmd.create_table: the example `with pk id(serial), name(text),
  email(text)` declares a 3-column COMPOUND primary key, not a PK
  plus regular columns (every `with pk` column is a key member,
  ADR-0005). Rewritten to a single-column PK + `add column` for the
  rest; what/concept aligned.
- cmd.save: `save as my-shop` does not parse — `save as` takes no
  inline name, it opens a path-entry prompt. Example -> `save as`;
  what no longer implies inline naming; added a temp-vs-named concept.
- cmd.import: target `shop-copy` does not parse — the `as <target>`
  slot is a NewName ident that rejects hyphens. -> `shop_copy`.
- err.foreign_key.child_side: dropped the bogus `on delete set
  null/cascade` remedy — that governs the parent direction; a
  child-side violation is fixed by inserting the parent first
  (matches the tier-1 hint).

Adds every_cmd_hint_example_parses_in_its_mode — a catalog-driven
guard that parses every hint.cmd.* example in its taught mode,
backstopping syntactic drift (it caught the save and import errors).
Registers the new hint.cmd.save.concept key.

docs: drop two stale "deferred" entries from CLAUDE.md — project
storage (export/import, --resume, input history, migration scaffold)
and m:n convenience (C4) are all implemented (ADR-0015/0045); record
the verification pass on requirements.md H2.

src/friendly/keys.rs

@@ -277,6 +277,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("hint.cmd.rebuild.concept", &[]),
     ("hint.cmd.save.what", &[]),
     ("hint.cmd.save.example", &[]),
+    ("hint.cmd.save.concept", &[]),
     ("hint.cmd.new.what", &[]),
     ("hint.cmd.new.example", &[]),
     ("hint.cmd.load.what", &[]),