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_length_test.go

@@ -152,11 +152,6 @@ func TestGetAdaptiveIDLength_Integration(t *testing.T) {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
 	
-	// Set id_mode to hash
-	if err := db.SetConfig(ctx, "id_mode", "hash"); err != nil {
-		t.Fatalf("Failed to set id_mode: %v", err)
-	}
-	
 	// Test default config (should use 4 chars for empty database)
 	conn, err := db.db.Conn(ctx)
 	if err != nil {