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/daemon_lock_test.go

@@ -1,3 +1,6 @@
+//go:build integration
+// +build integration
+
 package main
 
 import (