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
claude@clouddev1
4a5fd1b5c1