Move migrations to internal/db/migrations (closes #96)

REPO_POLICIES.md puts migrations in internal/db/migrations/ as
000_migration.sql and 001_schema.sql. The two files move there with
their contents unchanged. go:embed cannot reach outside its own
package, so internal/db/migrations has a small package that embeds
them, and internal/database reads them through its FS(). The database
package stays where CONVENTIONS.md puts it; moving it would change
existing test files in other packages.

The version still comes from the filename prefix, so a database that
has recorded versions 0 and 1 runs neither again. A new test applies
the migrations twice to one database file and checks that the second
run applies nothing.

Model: opus-5-5

internal/db/migrations/migrations.go

@@ -0,0 +1,15 @@
+// Package migrations provides the embedded SQL migration files.
+package migrations
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed *.sql
+var files embed.FS
+
+// FS returns the embedded filesystem containing the migration files.
+func FS() fs.FS {
+	return files
+}