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/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.
 `)
 }