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/autoimport.go

@@ -188,7 +188,7 @@ func importFromGit(ctx context.Context, dbFilePath string, store storage.Storage
 	// Note: SkipPrefixValidation allows mixed prefixes during auto-import
 	// (but now we set the prefix first, so CreateIssue won't use filename fallback)
 	opts := ImportOptions{
-		ResolveCollisions:  true,
+
 		DryRun:             false,
 		SkipUpdate:         false,
 		SkipPrefixValidation: true, // Auto-import is lenient about prefixes