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/completion.rs

@@ -146,6 +146,22 @@ pub fn candidates_at_cursor(
         return None;
     }
 
+    // Value-literal slot: at an empty-prefix position the only
+    // candidates we'd surface are `null`/`true`/`false` (the
+    // keyword literals — number / string-literal slots are
+    // descriptive labels, not Tab candidates). Surfacing those
+    // three is actively misleading — the user usually wants a
+    // number, a quoted string, or a date, and seeing just
+    // "null true false" implies those are *the* options. We
+    // suppress the keyword candidates here; the ambient_hint
+    // ladder falls through to a prose hint with format
+    // examples instead. Once the user starts typing a prefix
+    // (`n`, `tr`, `fa`) the normal keyword-completion path
+    // applies — the suppression only triggers at empty prefix.
+    if partial_prefix.is_empty() && is_value_literal_signature(&expected) {
+        return None;
+    }
+
     let lowered_prefix = partial_prefix.to_lowercase();
     let matches_prefix = |s: &str| s.to_lowercase().starts_with(&lowered_prefix);
 
@@ -290,6 +306,53 @@ fn strip_backticks(s: &str) -> Option<&str> {
     s.strip_prefix('`').and_then(|s| s.strip_suffix('`'))
 }
 
+/// Detect a value-literal expected-set signature. A value-literal
+/// slot is the only position where chumsky's expected-set
+/// simultaneously contains all five forms `null` / `true` /
+/// `false` / number / string literal. See the suppression rationale
+/// at the call site in `candidates_at_cursor`.
+fn is_value_literal_signature(expected: &[String]) -> bool {
+    let has = |needle: &str| expected.iter().any(|e| e == needle);
+    has("`null`") && has("`true`") && has("`false`") && has("number") && has("string literal")
+}
+
+/// `Some(prose)` when the cursor sits at an empty-prefix value-literal slot.
+///
+/// The hint panel surfaces format guidance (number, quoted text,
+/// date, datetime, bool, null) instead of the misleading "null
+/// true false" keyword-only candidate list.
+///
+/// Note: this is a stopgap until ADR-0023 lands schema-aware
+/// completion (which would surface format examples specific to
+/// the column's type — e.g. just the datetime format at a
+/// datetime column). Today the hint lists all valid literal
+/// shapes regardless of context.
+#[must_use]
+pub fn value_literal_hint_at_cursor(input: &str, cursor: usize) -> Option<String> {
+    let cursor = cursor.min(input.len());
+    let bytes = input.as_bytes();
+    let mut start = cursor;
+    while start > 0 {
+        let prev = bytes[start - 1];
+        if prev.is_ascii_alphanumeric() || prev == b'_' {
+            start -= 1;
+        } else {
+            break;
+        }
+    }
+    if start != cursor {
+        // Partial prefix is non-empty — keyword completion
+        // handles it (e.g. `n` → `null`).
+        return None;
+    }
+    let leading = &input[..start];
+    let expected = expected_set(leading);
+    if !is_value_literal_signature(&expected) {
+        return None;
+    }
+    Some(crate::t!("hint.value_literal_slot"))
+}
+
 /// What the user has typed in an identifier slot whose schema
 /// list contains nothing matching the prefix (ADR-0022 stage 8e
 /// + the user's #5).
@@ -680,6 +743,100 @@ mod tests {
     // need a separate probe mechanism (deferred — same shape as
     // the post-complete-parse gap for `--create-fk` etc.).
 
+    // ---- Value-literal slot suppression (round-6) -----------
+
+    #[test]
+    fn value_literal_slot_suppresses_keyword_candidates_at_empty_prefix() {
+        // After `insert into T values (` the parser's expected
+        // set contains null/true/false/number/string literal.
+        // The keyword pipeline would otherwise surface `null`,
+        // `true`, `false` as Tab candidates — actively
+        // misleading at a slot where the user is more likely
+        // entering a number / text / date. Suppress.
+        let cs = cands("insert into T values (", 22);
+        assert!(cs.is_empty(), "got misleading candidates {cs:?}");
+    }
+
+    #[test]
+    fn value_literal_slot_with_partial_prefix_still_completes() {
+        // Once the user types a prefix, normal keyword
+        // completion applies — `n` → `null`, `tr` → `true`,
+        // `fa` → `false`.
+        assert_eq!(
+            cands("insert into T values (n", 23),
+            vec!["null".to_string()],
+        );
+        assert_eq!(
+            cands("insert into T values (tr", 24),
+            vec!["true".to_string()],
+        );
+        assert_eq!(
+            cands("insert into T values (fa", 24),
+            vec!["false".to_string()],
+        );
+    }
+
+    #[test]
+    fn value_literal_slot_after_first_value_also_suppresses() {
+        // Comma-separated value positions all hit the same slot
+        // signature. `insert into T values (1, ` → expected:
+        // null/true/false/number/string. Suppress.
+        let cs = cands("insert into T values (1, ", 25);
+        assert!(cs.is_empty(), "got {cs:?}");
+    }
+
+    #[test]
+    fn update_set_value_slot_suppresses() {
+        // `update T set col=` is also a value-literal slot.
+        let cs = cands("update T set col=", 17);
+        assert!(cs.is_empty(), "got {cs:?}");
+    }
+
+    #[test]
+    fn where_value_slot_suppresses() {
+        // `where col=` is also a value-literal slot.
+        let cs = cands("delete from T where col=", 24);
+        assert!(cs.is_empty(), "got {cs:?}");
+    }
+
+    #[test]
+    fn value_literal_hint_fires_at_empty_value_slot() {
+        let hint = value_literal_hint_at_cursor("insert into T values (", 22);
+        let s = hint.expect("hint should fire at value-literal slot");
+        // Lists each literal form so the user sees the full set
+        // of valid inputs rather than just three keywords.
+        assert!(s.contains("number"), "got {s:?}");
+        assert!(s.contains("text") || s.contains("'"), "got {s:?}");
+        assert!(s.contains("true"), "got {s:?}");
+        assert!(s.contains("false") || s.contains("/false"), "got {s:?}");
+        assert!(s.contains("null"), "got {s:?}");
+        // Format examples for the cases users typically can't
+        // guess (date, datetime).
+        assert!(
+            s.contains("YYYY-MM-DD"),
+            "should include date format, got {s:?}",
+        );
+        assert!(
+            s.contains("HH:MM:SS"),
+            "should include datetime format, got {s:?}",
+        );
+    }
+
+    #[test]
+    fn value_literal_hint_does_not_fire_at_partial_prefix() {
+        // With a partial prefix the keyword-completion path
+        // handles it; the prose hint short-circuit only
+        // applies to empty-prefix positions.
+        assert!(value_literal_hint_at_cursor("insert into T values (n", 23).is_none());
+    }
+
+    #[test]
+    fn value_literal_hint_does_not_fire_at_keyword_slot() {
+        // Entry keyword position is not a value-literal slot.
+        assert!(value_literal_hint_at_cursor("", 0).is_none());
+        assert!(value_literal_hint_at_cursor("insert ", 7).is_none());
+    }
+
     #[test]
     fn show_offers_data_and_table_alphabetised() {
         let cs = cands("show ", 5);