fix(linter): support bare true/false and integer literals in rules:if:

GitLab CI expressions allow unquoted true, false, and integers as
comparison operands (all treated as their string representations):

  $GATEWAY_ENABLED == true    (equivalent to == "true")
  $FEATURE_FLAG == false      (equivalent to == "false")
  $PARALLEL == 4              (equivalent to == "4")
  $ENABLED == 1 / == 0

Previously these fell through to permissive true because parseValue
only recognised $VAR, "${VAR}", quoted strings, and null. Added:
  - true/false keyword branch → returns "true"/"false"
  - integer literal branch (digits only) → returns decimal string

All three new forms are correctly excluded from longer identifier
prefixes (identByte boundary check). Adds 8 new unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/cicontext/eval.go

@@ -231,8 +231,10 @@ func (p *exprParser) parseRegexRHS() (pat string, ok bool, permissive bool) {
 	return "", false, false
 }
 
-// parseValue reads $VAR, ${VAR}, "string", 'string', or null.
-// null and undefined variables both produce an empty string.
+// parseValue reads $VAR, ${VAR}, "string", 'string', null, true, false, or an
+// integer literal. null and undefined variables both produce an empty string.
+// true/false and integers produce their string representations (GitLab CI
+// compares all values as strings).
 func (p *exprParser) parseValue() (string, bool) {
 	p.skipWS()
 
@@ -254,12 +256,18 @@ func (p *exprParser) parseValue() (string, bool) {
 		return p.vars(name), true
 	}
 
-	// null keyword — must not be a prefix of a longer identifier.
-	if p.startsWith("null") {
-		end := p.pos + 4
-		if end >= len(p.s) || !isIdentByte(p.s[end]) {
-			p.pos += 4
-			return "", true // null → empty string
+	// Keywords and string literals must not be prefixes of longer identifiers.
+	for _, kw := range []struct{ tok, val string }{
+		{"null", ""},
+		{"true", "true"},
+		{"false", "false"},
+	} {
+		if p.startsWith(kw.tok) {
+			end := p.pos + len(kw.tok)
+			if end >= len(p.s) || !isIdentByte(p.s[end]) {
+				p.pos += len(kw.tok)
+				return kw.val, true
+			}
 		}
 	}
 
@@ -267,6 +275,15 @@ func (p *exprParser) parseValue() (string, bool) {
 		return p.parseStringLiteral()
 	}
 
+	// Integer literal — returned as its decimal string for string comparison.
+	if p.peek() >= '0' && p.peek() <= '9' {
+		start := p.pos
+		for p.pos < len(p.s) && p.s[p.pos] >= '0' && p.s[p.pos] <= '9' {
+			p.pos++
+		}
+		return p.s[start:p.pos], true
+	}
+
 	return "", false
 }