refactor: centralize and improve error handling and path construction

include/utils.h

@@ -0,0 +1,22 @@
+
+#ifndef KEYVIBE_UTILS_H
+#define KEYVIBE_UTILS_H
+
+#include <stddef.h>
+#include <sys/types.h>
+
+// Returns 1 on success, 0 on truncation/error
+int safe_snprintf(char *dst, size_t dst_sz, const char *fmt, ...);
+
+// fprintf to stderr with printf-style formatting. Returns 1 for convenient
+// `return errorf(...)` usage.
+int errorf(const char *fmt, ...);
+
+// Common runtime helpers
+const char *get_runtime_dir(void);
+void build_pidfile_path(char *buffer, size_t buflen);
+int read_pidfile(const char *path, pid_t *out_pid);
+int write_pidfile(const char *path, pid_t pid);
+int process_is_running(pid_t pid);
+
+#endif // KEYVIBE_UTILS_H