Remove vestigial --resolve-collisions flag

Hash-based IDs make collision resolution unnecessary. The flag was
already non-functional (handleCollisions returns error on collision
regardless of flag value).

Removed:
- --resolve-collisions flag from bd import
- ResolveCollisions field from ImportOptions and importer.Options
- All references in daemon, auto-import, and tests
- Updated error messages to reflect hash IDs don't collide

All import tests pass.

Amp-Thread-ID: https://ampcode.com/threads/T-47dfa0cc-bb71-4467-ac86-f0966a7c5d58
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/jsonl_integrity_test.go

@@ -214,7 +214,7 @@ func TestImportClearsExportHashes(t *testing.T) {
 	}
 	
 	opts := ImportOptions{
-		ResolveCollisions:    false,
+
 		DryRun:               false,
 		SkipUpdate:           false,
 		Strict:               false,