refactor: remove headless mode

Headless mode was half-built and untested. Agent-loop is a plugin
that runs interactively via tmux — there's no CI use case yet.
Removes --headless flag, timeout compatibility shim, output capture
logic, and LOOP_AGENT_TMPFILE handling. Cuts 82 lines from loop.sh.

install.sh

@@ -102,5 +102,5 @@ echo "  Next steps (inside Claude Code, in any project):"
 echo ""
 echo "    /agent-loop:run      # Single command — setup, plan, and run"
 echo ""
-echo "  Or run headless:    .loop/loop.sh"
+echo "  Or run directly:    .loop/loop.sh"
 echo ""