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.

docs/todo.md

@@ -96,12 +96,19 @@ Legend: ✅ done · 🚧 in progress · ⬜ not started
 
 ---
 
-## V2 — Linter (later)
-- `pkg/pgast`: `go-pgquery` (WASM, no cgo) wrapper → real PG AST.
-- `pkg/lint`: rule engine + packs — style/consistency, **migration safety** (locks, unsafe
-  ALTER/ADD COLUMN, non-CONCURRENTLY index, blocking constraints), naming, correctness.
-- `pkg/diagnostics`: shared diagnostic type (CLI + LSP).
-- `pgtidy lint` subcommand; `--fix` for autofixable rules.
+## ✅ V2 — Linter
+- `pkg/pgast`: `go-pgquery` (WASM, no cgo) wrapper → real PG AST. `FirstTokenOffset`
+  skips leading whitespace/comments for accurate line numbers.
+- `pkg/diagnostics`: `Diagnostic{RuleID, Severity, Message, File, Line, Col}`.
+- `pkg/lint`: `Engine`, `Rule` interface, `New()` with all built-ins:
+  - MIG001 CREATE INDEX without CONCURRENT
+  - MIG002 ALTER TABLE ADD COLUMN NOT NULL without DEFAULT
+  - MIG003 ALTER TABLE ADD CONSTRAINT FK/CHECK without NOT VALID
+  - COR001 SELECT *  |  COR002 UPDATE without WHERE  |  COR003 DELETE without WHERE
+  - NAM001/2/3 table/column/function names not snake_case (quoted identifiers only)
+- `pgtidy lint [--only=ID,...] [files...]`; exits 1 on findings, 2 on error.
+- Fixture SQL in `testdata/lint/`; 6 tests covering violations + clean fixtures.
+- _`--fix` for autofixable rules: future._
 
 ## V3 — LSP + VSCode (later)
 - `pkg/lsp`: formatting + range formatting, publishDiagnostics, codeAction quick-fixes.