Remove sequential ID code path (bd-aa744b)

- Removed nextSequentialID() and getIDMode() functions
- Removed issue_counters table from schema
- Made SyncAllCounters() a no-op for backward compatibility
- Simplified ID generation to hash-only (adaptive length)
- Removed id_mode config setting
- Removed sequential ID tests and migration code
- Updated CONFIG.md and AGENTS.md to remove sequential ID references

Follow-up bd-2a70 will remove obsolete test files and renumber command.

internal/storage/sqlite/hash_id_test.go

@@ -17,13 +17,10 @@ func TestHashIDGeneration(t *testing.T) {
 
 	ctx := context.Background()
 
-	// Set up database with prefix and hash mode
+	// Set up database with prefix
 	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
-	if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
-		t.Fatalf("Failed to set id_mode: %v", err)
-	}
 
 	// Create an issue - should get a hash ID
 	issue := &types.Issue{
@@ -143,13 +140,10 @@ func TestHashIDBatchCreation(t *testing.T) {
 
 	ctx := context.Background()
 
-	// Set up database with prefix and hash mode
+	// Set up database with prefix
 	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
-	if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
-		t.Fatalf("Failed to set id_mode: %v", err)
-	}
 
 	// Create multiple issues with similar content
 	issues := []*types.Issue{