fix(agent-loop): redirect stdin from /dev/null to prevent tmux PTY blocking

Without `< /dev/null`, claude detects the tmux PTY as stdin and blocks
waiting for user input that never arrives (the PTY never sends EOF).
The 3-second stdin-timeout only fires for pipe stdin, not TTY stdin.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/agent_loop.py

@@ -179,7 +179,7 @@ def _start_agent(prompt: str, session_name: str) -> str:
         f"claude --dangerously-skip-permissions"
         f" --name {shlex.quote(session_name)}"
         f" -p {shlex.quote(prompt)}"
-        f" 2>&1 | tee {shlex.quote(str(log_file))}"
+        f" < /dev/null 2>&1 | tee {shlex.quote(str(log_file))}"
     )
     subprocess.run(
         ["tmux", "new-session", "-d", "-s", session_name, "bash", "-c", shell_cmd],