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/friendly/keys.rs

@@ -133,6 +133,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         &["kind", "found"],
     ),
     ("hint.ambient_typing_name", &[]),
+    ("hint.value_literal_slot", &[]),
     (
         "hint.ambient_typing_name_then",
         &["next"],