feat(lint): add linter rules for migration and naming conventions

* Implement migration safety rules:
  - MIG001: Warn on CREATE INDEX without CONCURRENT.
  - MIG002: Warn on ALTER TABLE ADD COLUMN NOT NULL without DEFAULT.
  - MIG003: Warn on ALTER TABLE ADD CONSTRAINT without NOT VALID.
* Implement naming conventions rules:
  - NAM001: Warn on non-snake_case table names.
  - NAM002: Warn on non-snake_case column names in CREATE TABLE.
  - NAM003: Warn on non-snake_case function names.
* Add test fixtures for all new rules.

pkg/lint/lint_test.go

@@ -0,0 +1,125 @@
+package lint_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/hein/pgtidy/pkg/diagnostics"
+	"github.com/hein/pgtidy/pkg/lint"
+)
+
+func fixtureDir() string {
+	return filepath.Join("..", "..", "testdata", "lint")
+}
+
+func checkFile(t *testing.T, name string) []diagnostics.Diagnostic {
+	t.Helper()
+	src, err := os.ReadFile(filepath.Join(fixtureDir(), name))
+	if err != nil {
+		t.Fatalf("read %s: %v", name, err)
+	}
+	eng := lint.New()
+	diags, err := eng.Check(string(src), name)
+	if err != nil {
+		t.Fatalf("Check(%s): %v", name, err)
+	}
+	return diags
+}
+
+func ruleIDs(diags []diagnostics.Diagnostic) map[string]int {
+	m := make(map[string]int)
+	for _, d := range diags {
+		m[d.RuleID]++
+	}
+	return m
+}
+
+func TestMigrationViolations(t *testing.T) {
+	diags := checkFile(t, "migration_violations.sql")
+	ids := ruleIDs(diags)
+
+	cases := []struct {
+		id    string
+		count int
+	}{
+		{"MIG001", 1}, // one non-concurrent index
+		{"MIG002", 1}, // one NOT NULL + no DEFAULT
+		{"MIG003", 2}, // one FK + one CHECK without NOT VALID
+	}
+	for _, c := range cases {
+		if got := ids[c.id]; got != c.count {
+			t.Errorf("rule %s: want %d findings, got %d", c.id, c.count, got)
+		}
+	}
+}
+
+func TestMigrationClean(t *testing.T) {
+	diags := checkFile(t, "migration_clean.sql")
+	ids := ruleIDs(diags)
+	for _, id := range []string{"MIG001", "MIG002", "MIG003"} {
+		if n := ids[id]; n != 0 {
+			t.Errorf("rule %s: want 0 findings on clean fixture, got %d", id, n)
+		}
+	}
+}
+
+func TestCorrectnessViolations(t *testing.T) {
+	diags := checkFile(t, "correctness_violations.sql")
+	ids := ruleIDs(diags)
+
+	cases := []struct {
+		id    string
+		count int
+	}{
+		{"COR001", 1},
+		{"COR002", 1},
+		{"COR003", 1},
+	}
+	for _, c := range cases {
+		if got := ids[c.id]; got != c.count {
+			t.Errorf("rule %s: want %d findings, got %d", c.id, c.count, got)
+		}
+	}
+}
+
+func TestNamingViolations(t *testing.T) {
+	diags := checkFile(t, "naming_violations.sql")
+	ids := ruleIDs(diags)
+
+	cases := []struct {
+		id    string
+		count int
+	}{
+		{"NAM001", 1}, // UserAccounts
+		{"NAM002", 2}, // userId, emailAddress
+		{"NAM003", 1}, // GetUserById
+	}
+	for _, c := range cases {
+		if got := ids[c.id]; got != c.count {
+			t.Errorf("rule %s: want %d findings, got %d", c.id, c.count, got)
+		}
+	}
+}
+
+func TestParseError(t *testing.T) {
+	eng := lint.New()
+	diags, err := eng.Check("SELECT FROM WHERE", "test.sql")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(diags) == 0 || diags[0].RuleID != "PARSE" {
+		t.Errorf("expected a PARSE diagnostic, got %v", diags)
+	}
+}
+
+func TestCustomEngine(t *testing.T) {
+	eng := lint.Custom()
+	eng.Register(lint.New().Rules()[0]) // register first rule only
+	// Just confirm it doesn't panic and returns results.
+	diags, err := eng.Check("SELECT 1", "stdin")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	_ = diags
+}