fix: TOCTOU race in personal file delete + IndexError on whitespace cmd (#2228)

1. routes/personal_routes.py: os.path.exists() then os.remove() is a
   classic TOCTOU race — another request or cleanup can delete the
   file between the check and the remove, raising FileNotFoundError.
   Replace with try/except FileNotFoundError.

2. src/tool_implementations.py: cmd.split()[0] crashes with IndexError
   when cmd is a non-empty whitespace-only string (split() returns []).
   Guard with (cmd.split() or [''])[0].

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/tool_implementations.py

@@ -2662,7 +2662,7 @@ async def _cookbook_register_task(session_id: str, model: str, host: str,
     placeholder = (
         f"Launched via agent — waiting for tmux output…\n"
         f"  session: {session_id}\n"
-        f"  target:  {target}{cmd.split()[0] if cmd else ''}\n"
+        f"  target:  {target}{(cmd.split() or [''])[0] if cmd else ''}\n"
         f"  cmd:     {cmd[:200]}{'…' if len(cmd) > 200 else ''}"
     )
     tasks.append({