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/adaptive_e2e_test.go

@@ -18,13 +18,10 @@ func TestAdaptiveIDLength_E2E(t *testing.T) {
 	
 	ctx := context.Background()
 	
-	// Initialize with prefix and hash mode
+	// Initialize with prefix
 	if err := db.SetConfig(ctx, "issue_prefix", "test"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
-	if err := db.SetConfig(ctx, "id_mode", "hash"); err != nil {
-		t.Fatalf("Failed to set id_mode: %v", err)
-	}
 	
 	// Helper to create issue and verify ID length
 	createAndCheckLength := func(title string, expectedHashLen int) string {
@@ -124,9 +121,6 @@ func TestAdaptiveIDLength_CustomConfig(t *testing.T) {
 	if err := db.SetConfig(ctx, "issue_prefix", "test"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
-	if err := db.SetConfig(ctx, "id_mode", "hash"); err != nil {
-		t.Fatalf("Failed to set id_mode: %v", err)
-	}
 	
 	// Set stricter collision threshold (1%) and min length of 5
 	if err := db.SetConfig(ctx, "max_collision_prob", "0.01"); err != nil {