libtmux(fix[logging]): move cmd_str inside isEnabledFor guard

why: shlex.join() was computed on every fetch_objs call regardless of
log level. Per AGENTS.md, expensive computations for log extra values
should be guarded with isEnabledFor.
what:
- Move cmd_str = shlex.join(...) inside first isEnabledFor(DEBUG) block
- Lazily compute in second block if DEBUG was enabled between the two

Author: tony

src/libtmux/neo.py

@@ -306,9 +306,10 @@ def fetch_objs(
 
     tmux_cmds.append(f"-F{format_string}")
 
-    cmd_str = shlex.join([str(x) for x in tmux_cmds])
+    cmd_str: str | None = None
 
     if logger.isEnabledFor(logging.DEBUG):
+        cmd_str = shlex.join([str(x) for x in tmux_cmds])
         logger.debug(
             "tmux list queried",
             extra={
@@ -325,6 +326,8 @@ def fetch_objs(
     outputs = [parse_output(line) for line in proc.stdout]
 
     if logger.isEnabledFor(logging.DEBUG):
+        if cmd_str is None:
+            cmd_str = shlex.join([str(x) for x in tmux_cmds])
         logger.debug(
             "tmux list parsed",
             extra={