Make zparseopts section argument extraction more robust

[Copilot]

The zparseopts syntax -section:=section_arg populates an array with both option name (index 1) and value (index 2). Hardcoding section_arg[2] is fragile if zparseopts behavior changes.

Changes:

• Use section_arg[-1] to extract the last element instead of hardcoded index 2
• Validate array has at least 2 elements before access
• Return error when --section is provided without a value

# Before
if [[ ${#section_arg[@]} -gt 0 ]]; then
  local section_name="${section_arg[2]}"
  _run_single_section "$section_name"
fi

# After
if [[ ${#section_arg[@]} -gt 0 ]]; then
  if [[ ${#section_arg[@]} -lt 2 ]]; then
    echo "Error: --section requires a value" >&2
    return 1
  fi
  local section_name="${section_arg[-1]}"
  _run_single_section "$section_name"
fi

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