Hint: pedagogical Form-A pointer at Form B's first value slot

Handoff-12 §2.2: Form B `insert into T values (…)` silently skips
auto-generated columns from the value list, so a user who wants to
set a serial/shortid column explicitly could only discover Form A by
reading help. Now the hint at the first Form B value slot appends a
note naming the skipped column(s) and pointing at the explicit-column
form.

hint_resolution_at_input derives the skipped columns from the
post-walk WalkContext (Form B = no user_listed_columns + table has
serial/shortid columns) and reports them on HintResolution; the note
fires only at the first slot so it doesn't repeat at every comma.
ambient_hint composes it onto the per-column prose.

src/friendly/keys.rs

@@ -150,6 +150,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("hint.value_slot_shortid", &[]),
     ("hint.value_slot_text", &[]),
     ("hint.value_slot_for_column", &["column", "detail"]),
+    ("hint.value_slot_autogen_skipped", &["columns"]),
     // ---- Parse error rendering ----
     ("parse.available_commands", &["commands"]),
     ("parse.caret", &["padding"]),