Remove sequential ID generation and SyncAllCounters (bd-c7af, bd-8e05, bd-4c74)

- Removed SyncAllCounters() and all call sites (already no-op with hash IDs)
- Removed AllocateNextID() and getNextIDForPrefix() - sequential ID generation
- Removed collision remapping logic in internal/storage/sqlite/collision.go
- Removed rename collision handling in internal/importer/importer.go
- Removed branch-merge example (collision resolution no longer needed)
- Updated EXTENDING.md to remove counter sync examples

These were all deprecated code paths for sequential IDs that are obsolete
with hash-based IDs. Hash ID collisions are handled by extending the hash,
not by remapping to new sequential IDs.

cmd/bd/import_collision_test.go

@@ -711,10 +711,7 @@ func TestImportCounterSyncAfterHighID(t *testing.T) {
 		t.Fatalf("Failed to import high ID issue: %v", err)
 	}
 
-	// Step 4: Sync counters after import (mimics import command behavior)
-	if err := testStore.SyncAllCounters(ctx); err != nil {
-		t.Fatalf("Failed to sync counters: %v", err)
-	}
+	// REMOVED (bd-c7af): Counter sync - no longer needed with hash IDs
 
 	// Step 5: Create another auto-generated issue
 	// This should get bd-101 (counter should have synced to 100), not bd-4