Make the placeholder migration an immutable 0000_bootstrap

The runner skips versions already recorded by filename, so replacing the
contents of the applied 0001_init.sql placeholder with the real schema
would leave upgraded databases without the new tables. Rename it to
0000_bootstrap.sql, document it as a permanent no-op, and leave
0001_init.sql free for the v1 schema as a brand-new version.

Add a test that migrates with the embedded set, then adds a fixture
0001_init.sql and proves it applies on the existing database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/migrate_test.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"context"
+	"io/fs"
 	"strings"
 	"testing"
 	"testing/fstest"
@@ -173,7 +174,33 @@ func TestMigrateEmbeddedMigrations(t *testing.T) {
 	if err := Migrate(context.Background(), s.Writer, migrations.FS); err != nil {
 		t.Fatal(err)
 	}
-	if got := versions(migrationRows(t, s)); !strings.HasPrefix(got, "0001_init") {
-		t.Errorf("versions = %q, want 0001_init first", got)
+	if got := versions(migrationRows(t, s)); got != "0000_bootstrap" {
+		t.Errorf("versions = %q, want 0000_bootstrap", got)
+	}
+}
+
+func TestMigrateEmbeddedThenNewMigrationApplies(t *testing.T) {
+	ctx := context.Background()
+	s := openTestStore(t)
+	if err := Migrate(ctx, s.Writer, migrations.FS); err != nil {
+		t.Fatal(err)
+	}
+	bootstrap, err := fs.ReadFile(migrations.FS, "0000_bootstrap.sql")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// A later build adds 0001_init.sql alongside the unchanged bootstrap.
+	upgraded := fstest.MapFS{
+		"0000_bootstrap.sql": {Data: bootstrap},
+		"0001_init.sql":      {Data: []byte("CREATE TABLE later (id INTEGER PRIMARY KEY);")},
+	}
+	if err := Migrate(ctx, s.Writer, upgraded); err != nil {
+		t.Fatal(err)
+	}
+	if got := versions(migrationRows(t, s)); got != "0000_bootstrap,0001_init" {
+		t.Errorf("versions = %q, want 0000_bootstrap,0001_init", got)
+	}
+	if !tableExists(t, s, "later") {
+		t.Error("table from 0001_init was not created")
 	}
 }

migrations/0000_bootstrap.sql

@@ -0,0 +1,4 @@
+-- 0000_bootstrap: immutable no-op bootstrap migration. It lets the migrations
+-- embed compile and proves the runner end to end. Never edit or remove it:
+-- applied versions are recorded by filename and never re-run. The initial v1
+-- schema arrives as a new migration, 0001_init.sql (Feature 02 task 3).

migrations/0001_init.sql

@@ -1,2 +0,0 @@
--- 0001_init: placeholder so the migrations embed compiles.
--- Replaced by the full v1 schema in Feature 02 task 3.