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/diagnostics/diagnostics.go

@@ -0,0 +1,30 @@
+// Package diagnostics defines the shared Diagnostic type emitted by the linter
+// (and, later, the LSP). Both the CLI and the language server import this
+// package to avoid a circular dependency between pkg/lint and pkg/lsp.
+package diagnostics
+
+// Severity is the urgency of a diagnostic.
+type Severity string
+
+const (
+	SeverityError   Severity = "error"
+	SeverityWarning Severity = "warning"
+	SeverityHint    Severity = "hint"
+)
+
+// Diagnostic is a single lint finding.
+type Diagnostic struct {
+	// RuleID is the stable identifier for the rule that produced this finding
+	// (e.g. "MIG001").
+	RuleID string
+	// Severity is the urgency level.
+	Severity Severity
+	// Message is a human-readable description of the finding.
+	Message string
+	// File is the path to the source file, or "" for stdin.
+	File string
+	// Line is the 1-based line number of the finding.
+	Line int
+	// Col is the 1-based column number of the finding.
+	Col int
+}