Optimize test suite: split integration tests, add -short support

- Split slow importer integration tests into separate file
- Add t.Short() guards to 10 slow daemon tests
- Document test organization in TEST_OPTIMIZATION.md
- Fast tests now run in ~50s vs 3+ minutes
- Use 'go test -short ./...' for fast feedback

Amp-Thread-ID: https://ampcode.com/threads/T-29ae21ac-749d-43d7-bf0c-2c5f7a06ae76
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/test_helpers_test.go

@@ -9,6 +9,8 @@ import (
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 )
 
+const windowsOS = "windows"
+
 // newTestStore creates a SQLite store with issue_prefix configured (bd-166)
 // This prevents "database not initialized" errors in tests
 func newTestStore(t *testing.T, dbPath string) *sqlite.SQLiteStorage {