hint: replace misleading "null true false" suggestions at value slots

At value-literal slots (`insert into T values (`, `update T set
col=`, `where col=`, comma positions) the expected-token set
contains null/true/false/number/string-literal. The completion
engine was surfacing the three keyword candidates as Tab options
— actively misleading because the user is usually about to enter
a number, quoted text, or date, and seeing "null true false"
implies those are *the* options. User report (round-6 testing):
"especially not when I'm trying to insert a datetime value and
don't know the correct format for the literal".

Fix: detect the value-literal slot by its expected-set
fingerprint. Suppress Tab candidates at empty prefix. Surface a
prose hint listing all literal forms with format examples
('YYYY-MM-DD' for dates, 'YYYY-MM-DDTHH:MM:SS' for datetimes).
Once the user starts typing a prefix (n / tr / fa), normal
keyword completion still applies.

Schema-aware narrowing (show ONLY the datetime format at a
datetime column) waits on ADR-0023.

Tests: 769 -> 777 passing (+8). Clippy clean.

src/input_render.rs

@@ -188,6 +188,13 @@ pub fn ambient_hint(
             selected: None,
         });
     }
+    // Value-literal slot at empty prefix: candidates_at_cursor
+    // returned None (it suppresses null/true/false here to avoid
+    // implying they're the only options). Surface a prose hint
+    // that names all valid literal forms with format examples.
+    if let Some(prose) = crate::completion::value_literal_hint_at_cursor(input, cursor) {
+        return Some(AmbientHint::Prose(prose));
+    }
     // User typing into a NewName slot — show the friendlier
     // "type a name" hint rather than the technical "next: …"
     // that the post-consumed-partial parse would otherwise