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.

cmd/pgtidy/lint.go

@@ -0,0 +1,146 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/hein/pgtidy/pkg/diagnostics"
+	"github.com/hein/pgtidy/pkg/lint"
+)
+
+// cmdLint implements `pgtidy lint`. Reads one or more SQL files (or stdin) and
+// reports lint findings to stdout. Exits non-zero when any findings are found.
+//
+// Output format (one line per finding):
+//
+//	file:line:col: [RULEID] severity: message
+func cmdLint(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
+	var (
+		only  []string // --only=ID,ID rule filter
+		files []string
+	)
+	for _, a := range args {
+		switch {
+		case a == "-h" || a == "--help":
+			lintUsage(stdout)
+			return 0
+		case strings.HasPrefix(a, "--only="):
+			ids := strings.Split(strings.TrimPrefix(a, "--only="), ",")
+			for _, id := range ids {
+				if id = strings.TrimSpace(id); id != "" {
+					only = append(only, strings.ToUpper(id))
+				}
+			}
+		case len(a) > 1 && a[0] == '-':
+			fmt.Fprintf(stderr, "pgtidy lint: unknown flag %q\n", a)
+			return 2
+		default:
+			files = append(files, a)
+		}
+	}
+
+	eng := lint.New()
+
+	check := func(sql, file string) ([]diagnostics.Diagnostic, error) {
+		diags, err := eng.Check(sql, file)
+		if err != nil {
+			return nil, err
+		}
+		if len(only) > 0 {
+			filtered := diags[:0]
+			for _, d := range diags {
+				for _, id := range only {
+					if d.RuleID == id {
+						filtered = append(filtered, d)
+						break
+					}
+				}
+			}
+			diags = filtered
+		}
+		return diags, nil
+	}
+
+	printDiags := func(diags []diagnostics.Diagnostic) {
+		for _, d := range diags {
+			loc := d.File
+			if d.Line > 0 {
+				loc = fmt.Sprintf("%s:%d:%d", d.File, d.Line, d.Col)
+			}
+			if loc == "" {
+				loc = "stdin"
+			}
+			fmt.Fprintf(stdout, "%s: [%s] %s: %s\n", loc, d.RuleID, d.Severity, d.Message)
+		}
+	}
+
+	found := false
+
+	if len(files) == 0 {
+		src, err := io.ReadAll(stdin)
+		if err != nil {
+			fmt.Fprintf(stderr, "pgtidy: reading stdin: %v\n", err)
+			return 2
+		}
+		diags, err := check(string(src), "")
+		if err != nil {
+			fmt.Fprintf(stderr, "pgtidy: %v\n", err)
+			return 2
+		}
+		printDiags(diags)
+		if len(diags) > 0 {
+			found = true
+		}
+	} else {
+		for _, path := range files {
+			src, err := os.ReadFile(path)
+			if err != nil {
+				fmt.Fprintf(stderr, "pgtidy: %v\n", err)
+				return 2
+			}
+			diags, err := check(string(src), path)
+			if err != nil {
+				fmt.Fprintf(stderr, "pgtidy: %v\n", err)
+				return 2
+			}
+			printDiags(diags)
+			if len(diags) > 0 {
+				found = true
+			}
+		}
+	}
+
+	if found {
+		return 1
+	}
+	return 0
+}
+
+func lintUsage(w io.Writer) {
+	fmt.Fprint(w, `Usage: pgtidy lint [flags] [file ...]
+
+Read SQL from files (or stdin) and report lint findings.
+
+Flags:
+  --only=ID,...   comma-separated rule IDs to enable (default: all rules)
+  -h, --help      show this help
+
+Rules:
+  MIG001  CREATE INDEX without CONCURRENTLY blocks writes
+  MIG002  ALTER TABLE ADD COLUMN NOT NULL with no DEFAULT may rewrite table
+  MIG003  ALTER TABLE ADD CONSTRAINT without NOT VALID validates all rows
+  COR001  SELECT * is fragile; enumerate columns explicitly
+  COR002  UPDATE without WHERE modifies every row
+  COR003  DELETE without WHERE removes every row
+  NAM001  Table name is not snake_case
+  NAM002  Column name is not snake_case
+  NAM003  Function/procedure name is not snake_case
+
+Exit codes:
+  0  no findings
+  1  one or more findings
+  2  error (file not found, parse error, bad flag)
+`)
+}

cmd/pgtidy/main.go

@@ -1,5 +1,4 @@
-// Command pgtidy is the PgTidy CLI: a PostgreSQL formatter (and, later, linter)
-// and LSP server. Today it provides the `fmt` subcommand.
+// Command pgtidy is the PgTidy CLI: a PostgreSQL formatter and linter.
 package main
 
 import (
@@ -23,6 +22,8 @@ func run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 	switch args[0] {
 	case "fmt", "format":
 		return cmdFmt(args[1:], stdin, stdout, stderr)
+	case "lint":
+		return cmdLint(args[1:], stdin, stdout, stderr)
 	case "version", "--version", "-v":
 		fmt.Fprintf(stdout, "pgtidy %s\n", version)
 		return 0
@@ -40,13 +41,20 @@ func usage(w io.Writer) {
 	fmt.Fprint(w, `pgtidy — PostgreSQL formatter and linter
 
 Usage:
-  pgtidy fmt [flags] [files...]   Format SQL/PL-pgSQL (stdin if no files)
-  pgtidy version                  Print version
-  pgtidy help                     Show this help
+  pgtidy fmt  [flags] [files...]   Format SQL/PL-pgSQL (stdin if no files)
+  pgtidy lint [flags] [files...]   Lint SQL (stdin if no files)
+  pgtidy version                   Print version
+  pgtidy help                      Show this help
 
 fmt flags:
   -w, --write    Rewrite files in place
   -l, --list     List files whose formatting differs (no writes)
+  -d, --diff     Print unified diff of changes
       --check    Exit non-zero if any input is not already formatted (CI)
+
+lint flags:
+  --only=ID,...  Enable only the specified rule IDs (comma-separated)
+
+Run 'pgtidy lint --help' for the full rule list.
 `)
 }