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/input_render.rs

@@ -237,7 +237,8 @@ pub fn ambient_hint(
             // candidate completion (e.g. `n` → `null`) applies.
             if cursor_partial_is_empty(input, cursor) {
                 let detail = crate::friendly::translate(key, &[]);
-                let composed = match resolution.and_then(|r| r.column) {
+                let resolution = resolution.expect("matched on resolution.mode");
+                let mut composed = match resolution.column {
                     Some(column) => crate::t!(
                         "hint.value_slot_for_column",
                         column = column,
@@ -245,6 +246,25 @@ pub fn ambient_hint(
                     ),
                     None => detail,
                 };
+                // Form B pedagogical note: when the first value
+                // slot of `insert into T values (…)` is reached
+                // and T has auto-generated columns the value list
+                // skips, point the user at the explicit-column
+                // form so the skipped column is discoverable
+                // (handoff-12 §2.2).
+                if !resolution.form_b_autogen_skipped.is_empty() {
+                    let columns = resolution
+                        .form_b_autogen_skipped
+                        .iter()
+                        .map(|c| format!("`{c}`"))
+                        .collect::<Vec<_>>()
+                        .join(", ");
+                    composed.push(' ');
+                    composed.push_str(&crate::t!(
+                        "hint.value_slot_autogen_skipped",
+                        columns = columns
+                    ));
+                }
                 return Some(AmbientHint::Prose(composed));
             }
         }