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>

internal/importer/importer.go

@@ -15,7 +15,6 @@ import (
 
 // Options contains import configuration
 type Options struct {
-	ResolveCollisions    bool // Auto-resolve collisions by remapping to new IDs
 	DryRun               bool // Preview changes without applying them
 	SkipUpdate           bool // Skip updating existing issues (create-only mode)
 	Strict               bool // Fail on any error (dependencies, labels, etc.)